fix: add timestamp to trigger immediately signal

[spkane31]

adding a timestamp to TriggerImmediatelyRequest message

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.

Covered by existing functional tests

Potential risks

[CLAassistant]

All committers have signed the CLA.

[yuandrew]

We don't need to manually pull in updates from the api repo, we have a GH action that runs when updates are merged to main that will automatically update api-go https://github.com/temporalio/api/actions/workflows/trigger-api-go-update.yml