Add the ability to produce an unsigned altair block

 - added `/eth/v2/validator/blocks/{slot}` route
 - added a note on the v1 endpoint that it only supports phase0 block production

 Added a note in the description of v2 to indicate the list of supported blocks will change over time as forks progress, hopefully setting the expectation that we're not creating a new api each time a fork happens.

apis/validator/block.v2.yaml

@@ -0,0 +1,62 @@
+get:
+  tags:
+    - ValidatorRequiredApi
+    - Validator
+  operationId: "produceBlockV2"
+  summary: "Produce a new block, without signature."
+  description: |
+    Requests a beacon node to produce a valid block, which can then be signed by a validator.
+
+    Metadata in the response indicates the type of block produced, and the supported types of block
+    will be added to as forks progress.
+  parameters:
+    - name: slot
+      in: path
+      required: true
+      description: "The slot for which the block should be proposed."
+      schema:
+        $ref: "../../beacon-node-oapi.yaml#/components/schemas/Uint64"
+    - name: randao_reveal
+      in: query
+      required: true
+      description: "The validator's randao reveal value."
+      schema:
+        $ref: '../../beacon-node-oapi.yaml#/components/schemas/Signature'
+    - name: graffiti
+      in: query
+      required: false
+      description: "Arbitrary data validator wants to include in block."
+      schema:
+        $ref: '../../beacon-node-oapi.yaml#/components/schemas/Graffiti'
+  responses:
+    "200":
+      description: Success response
+      content:
+        application/json:
+          schema:
+            title: ProduceBlockV2Response
+            type: object
+            properties:
+              version:
+                type: string
+                enum: [ phase0, altair ]
+                example: "phase0"
+              data:
+                oneOf:
+                 - $ref: '../../beacon-node-oapi.yaml#/components/schemas/BeaconBlock'
+                 - $ref: "../../beacon-node-oapi.yaml#/components/schemas/Altair.BeaconBlock"
+    "400":
+      description: "Invalid block production request"
+      content:
+        application/json:
+          schema:
+            $ref: "../../beacon-node-oapi.yaml#/components/schemas/ErrorMessage"
+          examples:
+            InvalidRequest:
+              value:
+                code: 400
+                message: "Invalid request to produce a block"
+    "500":
+      $ref: '../../beacon-node-oapi.yaml#/components/responses/InternalError'
+    "503":
+      $ref: '../../beacon-node-oapi.yaml#/components/responses/CurrentlySyncing'

apis/validator/block.yaml

@@ -4,7 +4,10 @@ get:
     - Validator
   operationId: "produceBlock"
   summary: "Produce a new block, without signature."
-  description: "Requests a beacon node to produce a valid block, which can then be signed by a validator."
+  description: |
+    Requests a beacon node to produce a valid block, which can then be signed by a validator.
+
+    __NOTE__: Supports only phase0 blocks.
   parameters:
     - name: slot
       in: path

beacon-node-oapi.yaml

@@ -120,6 +120,8 @@ paths:
     $ref: "./apis/validator/duties/proposer.yaml"
   /eth/v1/validator/blocks/{slot}:
     $ref: "./apis/validator/block.yaml"
+  /eth/v2/validator/blocks/{slot}:
+      $ref: "./apis/validator/block.v2.yaml"
   /eth/v1/validator/attestation_data:
     $ref: "./apis/validator/attestation_data.yaml"
   /eth/v1/validator/aggregate_attestation:
@@ -207,6 +209,8 @@ components:
       $ref: './types/http.yaml#/IndexedErrorMessage'
     Altair.SignedBeaconBlock:
       $ref: './types/altair/block.yaml#/Altair/SignedBeaconBlock'
+    Altair.BeaconBlock:
+      $ref: './types/altair/block.yaml#/Altair/BeaconBlock'
     Altair.BeaconState:
       $ref: './types/altair/state.yaml#/Altair/BeaconState'
     Altair.SyncCommitteeSubscription: