Run clients with ephemeral applications on the global event loop thread

[zanieb]

When a client is hooked to an ephemeral application, its moved to the global event loop instead of the event loop it is created on. We do this by sending the underlying httpx client's context manager to the global event loop and patch the client.request method to send coroutines to the other thread. This is a good stepping stone towards providing a singleton client that runs on the global event loop as well as towards providing a synchronous client.

Enabling this only for the ephemeral application is motivated by #9855 which requires SQLAlchemy to be managed on a separate thread than user code.

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[zanieb]

This is:

1. A possible way to fix issues in Run tasks on the main thread #9855
2. A path to a singleton client with a synchronous interface Allow Prefect client to be used from synchronous contexts #9695

This pull request appears to:

1. Introduce a large performance penalty in tests; this would likely not be seen by users but we are spawning a lot of extra threads in tests since we use a lot of ephemeral client instances.
2. Cause intermittent deadlocks

It's unclear what the best path forward is, but a more holistic approach will need to be taken.

@jakekaplan is best suited to take over this work.

[github-actions]

This pull request is stale because it has been open 60 days with no activity. To keep this pull request open remove stale label or comment.

[zanieb]

Closing this as I am not working on it and a new pull request will need to be opened if someone else takes it up.