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

.gitmodules

@@ -1,6 +1,7 @@
 [submodule "proto/api"]
 	path = proto/api
 	url = https://github.com/temporalio/api
+	branch = release/v1.50.x
 [submodule "proto/api-cloud"]
 	path = proto/api-cloud
 	url = https://github.com/temporalio/api-cloud