Add start delay field to StartActivityExecutionRequest (#767)

**What changed?**
Added a start_delay field to StartActivityExecutionRequest in the proto
definition and updated the OpenAPI specs accordingly.

**Why?**
To allow callers to specify a delay before the first standalone activity
task is dispatched, matching the existing start delay capability
available for workflow-started activities.

Author: fretz12

openapi/openapiv2.json

@@ -11298,6 +11298,10 @@
         "onConflictOptions": {
           "$ref": "#/definitions/apiCommonV1OnConflictOptions",
           "description": "Options for handling conflicts when using ACTIVITY_ID_CONFLICT_POLICY_USE_EXISTING."
+        },
+        "startDelay": {
+          "type": "string",
+          "description": "Time to wait before dispatching the first activity task. This delay is not applied to retry attempts."
         }
       }
     },

openapi/openapiv3.yaml

@@ -15430,6 +15430,10 @@ components:
           allOf:
             - $ref: '#/components/schemas/OnConflictOptions'
           description: Options for handling conflicts when using ACTIVITY_ID_CONFLICT_POLICY_USE_EXISTING.
+        startDelay:
+          pattern: ^-?(?:0|[1-9][0-9]{0,11})(?:\.[0-9]{1,9})?s$
+          type: string
+          description: Time to wait before dispatching the first activity task. This delay is not applied to retry attempts.
     StartActivityExecutionResponse:
       type: object
       properties:

temporal/api/workflowservice/v1/request_response.proto

@@ -3001,6 +3001,8 @@ message StartActivityExecutionRequest {
     repeated temporal.api.common.v1.Link links = 20;
     // Options for handling conflicts when using ACTIVITY_ID_CONFLICT_POLICY_USE_EXISTING.
     temporal.api.common.v1.OnConflictOptions on_conflict_options = 21;
+    // Time to wait before dispatching the first activity task. This delay is not applied to retry attempts.
+    google.protobuf.Duration start_delay = 22;
 }
 
 message StartActivityExecutionResponse {