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.

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**Breaking changes**

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right after this PR was merged. -->
[**Server PR**](temporalio/temporal#7968)

Author: spkane31