Add more state to the pending activity state (#565)

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<!-- Describe what has changed in this PR -->
**What changed?**
Add PAUSED/PAUSE_REQUESTED states to the PendictActivityState enum.

<!-- Tell your future self why have you made these changes -->
**Why?**
To easily differentiate between "activity is paused on server and still
running on worker" and "activity is paused on server and not running on
worker". This is the result of our discussion for activity commands.

In old terms:
"state == PAUSED" is "state = SCHEDULLED and paused_flag = true"
"state == PAUSE_REQUESTED" is "state = STARTED and paused_flag = true"

Author: ychebotarev

Diff for: openapi/openapiv2.json

@@ -11608,9 +11608,12 @@
         "PENDING_ACTIVITY_STATE_UNSPECIFIED",
         "PENDING_ACTIVITY_STATE_SCHEDULED",
         "PENDING_ACTIVITY_STATE_STARTED",
-        "PENDING_ACTIVITY_STATE_CANCEL_REQUESTED"
+        "PENDING_ACTIVITY_STATE_CANCEL_REQUESTED",
+        "PENDING_ACTIVITY_STATE_PAUSED",
+        "PENDING_ACTIVITY_STATE_PAUSE_REQUESTED"
       ],
-      "default": "PENDING_ACTIVITY_STATE_UNSPECIFIED"
+      "default": "PENDING_ACTIVITY_STATE_UNSPECIFIED",
+      "title": "- PENDING_ACTIVITY_STATE_PAUSED: PAUSED means activity is paused on the server, and is not running in the worker\n - PENDING_ACTIVITY_STATE_PAUSE_REQUESTED: PAUSE_REQUESTED means activity is currently running on the worker, but paused on the server"
     },
     "v1PendingChildExecutionInfo": {
       "type": "object",

Diff for: openapi/openapiv3.yaml

@@ -8715,6 +8715,8 @@ components:
             - PENDING_ACTIVITY_STATE_SCHEDULED
             - PENDING_ACTIVITY_STATE_STARTED
             - PENDING_ACTIVITY_STATE_CANCEL_REQUESTED
+            - PENDING_ACTIVITY_STATE_PAUSED
+            - PENDING_ACTIVITY_STATE_PAUSE_REQUESTED
           type: string
           format: enum
         heartbeatDetails:

Diff for: temporal/api/enums/v1/workflow.proto

@@ -106,6 +106,10 @@ enum PendingActivityState {
     PENDING_ACTIVITY_STATE_SCHEDULED = 1;
     PENDING_ACTIVITY_STATE_STARTED = 2;
     PENDING_ACTIVITY_STATE_CANCEL_REQUESTED = 3;
+    // PAUSED means activity is paused on the server, and is not running in the worker
+    PENDING_ACTIVITY_STATE_PAUSED = 4;
+    // PAUSE_REQUESTED means activity is currently running on the worker, but paused on the server
+    PENDING_ACTIVITY_STATE_PAUSE_REQUESTED = 5;
 }
 
 enum PendingWorkflowTaskState {