Generalized handling of link objects

[maurei]

The current state of the OpenApi integration has a hardcoded configuration for where and when links are included in schemas.

In JADNC this can be configured by:

• toggling JsonApiOptions.DefaultPageSize (enabling/disabling paging links)
  • This wouldn't be correct. Even when pagination is turned off by default, it can still be activated per request using the page[size] query string parameter.
• LinkTypes cascading configuration

These settings should be taken into account when links are included or excluded in the generation of schemas.

[bkoelman]

Should include an end-to-end test for describedby with a non-default path (see https://github.com/json-api-dotnet/JsonApiDotNetCore/blob/openapi/docs/usage/openapi.md#customizing-the-route-template).

[bkoelman]

Should include a test for the following scenario:

GET /api/todoItems/1/assignee HTTP/1.1

{
  "data": {
    "type": "people",
    "id": "1",
    "attributes": {
    },
    "relationships": {
    },
    "links": {
      "self": "/api/people/1" // <---
    }
  }
}

Where the marked link is omitted from the response when the /api/people/1 endpoint is unavailable.

Edit: Based on the insights gained from implementing #1518 (ie not hiding as many links as possible), this test doesn't need to be added. Hides_links_for_unregistered_controllers already verifies the link value is null in a similar case, which is unrelated to OpenAPI.