Change REST API to not work with encoded subjects

[zepatrik]

Describe the bug

The REST handler returns and expects subjects encoded. This disallows to put some characters into the subject when using REST, but it works over gRPC. The APIs should however be compatible and work with the same inputs/outputs.
This is a follow up of #661.

Reproducing the bug

Steps to reproduce the behavior:

			rts := []*relationtuple.InternalRelationTuple{
				{
					Namespace: nspace.Name,
					Object:    "group:B",
					Relation:  "member",
					Subject: &relationtuple.SubjectSet{
						Namespace: nspace.Name,
						Object:    "group:A",
						Relation:  "member",
					},
				},
				{
					Namespace: nspace.Name,
					Object:    "@all",
					Relation:  "member",
					Subject: &relationtuple.SubjectID{ID: "this:will#be interpreted:as a@subject set"},
				},
			}

work as expected over gRPC, but give 400 and 404 over REST.

Expected behavior

Parity between the API interfaces.

Breaking change

We will have to change the schema of the JSON data:

{
  "$schema": "../../../.schema/relation_tuple.schema.json",
  "namespace": "videos",
  "object": "/cats/1.mp4",
  "relation": "owner",
  "subject": "videos:/cats#owner"
}

will become

{
  "$schema": "../../../.schema/relation_tuple.schema.json",
  "namespace": "videos",
  "object": "/cats/1.mp4",
  "relation": "owner",
  "subject": {
    "namespace": "videos",
    "object": "/cats",
    "relation": "owner"
  }
}

[zepatrik]

Outdated proposal

Proposal for the new relationtuple schema:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "additionalProperties": false,
  "required": ["namespace", "relation", "object", "subject"],
  "properties": {
    "$schema": {
      "type": "string",
      "format": "uri-reference",
      "description": "Add this to allow defining the schema, useful for IDE integration"
    },
    "namespace": {
      "type": "string",
      "description": "The namespace of the object and relation in this tuple."
    },
    "relation": {
      "type": "string",
      "description": "The relation of the object and subject."
    },
    "object": {
      "type": "string",
      "description": "The object affected by this relation."
    },
    "subject": {
      "oneOf": [
        {
          "type": "object",
          "description": "The subject set affected by this relation.",
          "properties": {
            "namespace": {
              "type": "string",
              "description": "The namespace of the object and relation in this subject set."
            },
            "relation": {
              "type": "string",
              "description": "The relation of this subject set."
            },
            "object": {
              "type": "string",
              "description": "The object referenced in this subject set."
            }
          },
          "additionalProperties": false,
          "required": ["namespace", "relation", "object"]
        },
        {
          "type": "string",
          "description": "The subject ID affected by this relation."
        }
      ]
    }
  }
}

with the changes

     "subject": {
       "oneOf": [
         {
-          "type": "string",
-          "pattern": "^.*:.*#.*$",
-          "description": "The subject set affected by this relation. Uses the encoding of \"<namespace>:<object>#<relation>\"."
+          "type": "object",
+          "description": "The subject set affected by this relation.",
+          "properties": {
+            "namespace": {
+              "type": "string",
+              "description": "The namespace of the object and relation in this subject set."
+            },
+            "relation": {
+              "type": "string",
+              "description": "The relation of this subject set."
+            },
+            "object": {
+              "type": "string",
+              "description": "The object referenced in this subject set."
+            }
+          },
+          "additionalProperties": false,
+          "required": ["namespace", "relation", "object"]
         },
         {
           "type": "string",
-          "description": "The subject affected by this relation. Use \"<namespace>:<object>#<relation>\" to describe a subject set.",
-          "not": {
-            "pattern": "^.*:.*#.*$"
-          }
+          "description": "The subject ID affected by this relation."
         }
       ]
     }

[zepatrik]

As per discussion with @aeneasr, this will become

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "additionalProperties": false,
  "allOf": [
    {
      "required": [
        "namespace",
        "relation",
        "object"
      ],
      "properties": {
        "$schema": {
          "type": "string",
          "format": "uri-reference",
          "description": "Add this to allow defining the schema, useful for IDE integration"
        },
        "namespace": {
          "type": "string",
          "description": "The namespace of the object and relation in this tuple."
        },
        "object": {
          "type": "string",
          "description": "The object affected by this relation."
        },
        "relation": {
          "type": "string",
          "description": "The relation of the object and subject."
        }
      }
    },
    {
      "oneOf": [
        {
          "required": [
            "subject_id"
          ],
          "properties": {
            "subject_id": {
              "type": "string",
              "description": "The subject ID affected by this relation."
            }
          }
        },
        {
          "required": [
            "subject_set"
          ],
          "properties": {
            "subject_set": {
              "type": "object",
              "description": "The subject set affected by this relation.",
              "properties": {
                "namespace": {
                  "type": "string",
                  "description": "The namespace of the object and relation in this subject set."
                },
                "object": {
                  "type": "string",
                  "description": "The object referenced in this subject set."
                },
                "relation": {
                  "type": "string",
                  "description": "The relation of this subject set."
                }
              },
              "additionalProperties": false,
              "required": [
                "namespace",
                "relation",
                "object"
              ]
            }
          }
        }
      ]
    }
  ]
}