Added worker for Snowflake's Snowpark Container Services

[bjorhn]

This PR adds a new worker to the prefect-snowflake integration. The worker can run service jobs in Snowflake's Snowpark Container Services. Closes #15674

A few things worth mentioning:

• Snowflake's Python API currently doesn't support running async service jobs, so I'm temporarily running jobs using SQL commands instead. I will submit a PR to resolve this as soon as Snowflake adds the required features to their Python API.
• Most of the code was hacked together based on the official "custom worker" tutorial and copying things over from the Azure and Kubernetes workers. The same goes for the unit tests.
• Since a lot of the structure was copied over, let me know if I misunderstood how something should be done. For example, am I using run_sync_in_worker_thread correctly?
• I don't have a lot of experience creating unit tests in Python, so I've done the bare minimum with regards to unit testing.

I don't know the best way to test the worker after moving the modules over from our company repo to the Prefect repo. Perhaps someone could test it for me, or leave instructions in a comment. The code should be the same as it were before, it's just some import paths that have changed.

There are two bugs I've bumped into along the way which I don't think are caused by my code. I don't know if these issues only affect custom workers, but I think so:

• After creating a custom worker pool and changing any of the settings, you can no longer edit the pool using the GUI. See Base Job Configuration section (UI) of custom work pool disappears when adding a block reference #15678
• If I make changes to some of the settings in the pool, such as the Snowflake credentials, it seems like they don't apply until the pool is re-created.

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[bjorhn]

Is anything else required for this to be merged?

[desertaxle]

Is anything else required for this to be merged?

Hey @bjorhn! Apologies for the delay in review. Could you share some instructions for how to test out this worker? I haven't used Snowpark Container Service before, so some guides or documentation would be very helpful!