Refactor: Inject Client

[aaazzam]

Prefect's inject_client decorator wraps an async function with an Optional[PrefectClient] kwarg, injecting an inferred or new client if none is provided.

This PR refactors the get_or_infer_client logic to a separate function. This will makes type-hinting much easier. Getting static type checkers to acknowledge that @inject_client is providing a default to an optional kwarg is hard.

Consider the _get_variable_by_name utility, which fails basic type hinting:

With get_or_infer_client factored out, we recover better type hinting:

Checklist

• This pull request includes tests or only affects documentation.
• This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.

For new functions or classes in the Python SDK:

• This pull request includes helpful docstrings.

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[aaazzam]

woof my fears are realized: the in-function imports were for avoiding circular imports

[aaazzam]

I've changed the devex to be get_or_create and a la Django it returns a tuple of the client and whether a new context was created

[chrisguidry]

🔥 Thank you!