add activity_paused flag to ByID heartbeat response (#517)

<!-- Describe what has changed in this PR -->
**What changed?**
Add "activity_paused" flag to RecordActivityTaskHeartbeatByIdResponse.

<!-- Tell your future self why have you made these changes -->
**Why?**
1. It is needed to let long-running activity know that activity was
paused on the server.
2. Because it is missing in RecordActivityTaskHeartbeatByIdResponse but
present in RecordActivityTaskHeartbeatResponse

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
No.

Author: ychebotarev

Diff for: openapi/openapiv2.json

@@ -10537,6 +10537,10 @@
         "cancelRequested": {
           "type": "boolean",
           "description": "Will be set to true if the activity has been asked to cancel itself. The SDK should then\nnotify the activity of cancellation if it is still running."
+        },
+        "activityPaused": {
+          "type": "boolean",
+          "description": "Will be set to true if the activity is paused."
         }
       }
     },

Diff for: openapi/openapiv3.yaml

@@ -7906,6 +7906,9 @@ components:
           description: |-
             Will be set to true if the activity has been asked to cancel itself. The SDK should then
              notify the activity of cancellation if it is still running.
+        activityPaused:
+          type: boolean
+          description: Will be set to true if the activity is paused.
     RecordActivityTaskHeartbeatRequest:
       type: object
       properties:

Diff for: temporal/api/workflowservice/v1/request_response.proto

@@ -518,6 +518,9 @@ message RecordActivityTaskHeartbeatByIdResponse {
     // Will be set to true if the activity has been asked to cancel itself. The SDK should then
     // notify the activity of cancellation if it is still running.
     bool cancel_requested = 1;
+
+    // Will be set to true if the activity is paused.
+    bool activity_paused = 2;
 }
 
 message RespondActivityTaskCompletedRequest {