bug: add timestamp to TriggerImmediatelyRequest (#606)

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<!-- Describe what has changed in this PR -->
**What changed?**
adding a timestamp to `TriggerImmediatelyRequest` message

<!-- Tell your future self why have you made these changes -->
We can make the wft “more idempotent”: put the desired trigger time in
the trigger immediately request itself, so it goes in the signal
payload, so all wfts attempt to start the target with the same id.


<!-- 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**](temporalio/temporal#7968)

Author: spkane31

openapi/openapiv2.json

@@ -14748,6 +14748,11 @@
         "overlapPolicy": {
           "$ref": "#/definitions/v1ScheduleOverlapPolicy",
           "description": "If set, override overlap policy for this one request."
+        },
+        "scheduledTime": {
+          "type": "string",
+          "format": "date-time",
+          "description": "Timestamp used for the identity of the target workflow.\nIf not set the default value is the current time."
         }
       }
     },

openapi/openapiv3.yaml

@@ -11907,6 +11907,12 @@ components:
           type: string
           description: If set, override overlap policy for this one request.
           format: enum
+        scheduledTime:
+          type: string
+          description: |-
+            Timestamp used for the identity of the target workflow.
+             If not set the default value is the current time.
+          format: date-time
     TriggerWorkflowRuleRequest:
       type: object
       properties:

temporal/api/schedule/v1/message.proto

@@ -278,6 +278,10 @@ message ScheduleState {
 message TriggerImmediatelyRequest {
     // If set, override overlap policy for this one request.
     temporal.api.enums.v1.ScheduleOverlapPolicy overlap_policy = 1;
+    
+    // Timestamp used for the identity of the target workflow.
+    // If not set the default value is the current time.
+    google.protobuf.Timestamp scheduled_time = 2;
 }
 
 message BackfillRequest {