Introduce feature flag to rollback apply trial in background

What does this MR do and why?

introduces a feature flag to drive placing trial creation in the background - this is being done to try and diagnose an issue we are seeing in production in the logs with applying trials in the background. This will basically revert us to the state of applying trial in a foreground that was removed in !96807 (merged)

There is business impact here for trials as they can't be created during registration right now, so we are looking to get this code in as soon as possible.

MR acceptance checklist

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Related to #373270 (closed)