Add link field to Callback (#576)

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<!-- Describe what has changed in this PR -->
**What changed?**
Add `links` field to `Callback`.

<!-- Tell your future self why have you made these changes -->
**Why?**
Be able to explicitly associate a callback with links.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**


<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
**Server PR**

Author: rodrigozhou

openapi/openapiv2.json

@@ -8952,6 +8952,14 @@
         },
         "internal": {
           "$ref": "#/definitions/CallbackInternal"
+        },
+        "links": {
+          "type": "array",
+          "items": {
+            "type": "object",
+            "$ref": "#/definitions/apicommonv1Link"
+          },
+          "description": "Links associated with the callback. It can be used to link to underlying resources of the\ncallback."
         }
       },
       "description": "Callback to attach to various events in the system, e.g. workflow run completion."

openapi/openapiv3.yaml

@@ -6508,6 +6508,13 @@ components:
           $ref: '#/components/schemas/Callback_Nexus'
         internal:
           $ref: '#/components/schemas/Callback_Internal'
+        links:
+          type: array
+          items:
+            $ref: '#/components/schemas/Link'
+          description: |-
+            Links associated with the callback. It can be used to link to underlying resources of the
+             callback.
       description: Callback to attach to various events in the system, e.g. workflow run completion.
     CallbackInfo:
       type: object

temporal/api/common/v1/message.proto

@@ -211,6 +211,10 @@ message Callback {
         Nexus nexus = 2;
         Internal internal = 3;
     }
+
+    // Links associated with the callback. It can be used to link to underlying resources of the
+    // callback.
+    repeated Link links = 100;
 }
 
 // Link can be associated with history events. It might contain information about an external entity