Add support for custom flow decorators to prefect deploy

[desertaxle]

This PR alters how the client loads a flow from source code when dependencies are missing. Previously, the client would attempt to retrieve flow metadata from the @flow decorator and generate a parameter schema for the decorated function by traversing the abstract syntax tree (AST). This approach prevents the use of custom decorators that wrap the @flow decorator.

This PR instead adds a safe_load_flow_from_entrypoint function which is capable of loading the full flow after making modifications to the AST. safe_load_flow_from_entrypoint will use safe_load_namespace to avoid raising exceptions when dependencies are not present on the current machine when loading a flow. If the flow depends on types or objects from missing dependencies, the AST will be updated to remove the annotations, causing loading to fail. This can generate a different parameter schema if the necessary dependencies are present or not.

safe_load_flow_from_entrypoint is used as a fallback within load_flow_from_entrypoint, so the behavior should not be altered in most cases.

I recognize this implementation is complex and understand this PR might be rejected. If it is accepted, or you decide to go an alternative route without AST parsing, much of the old AST parsing implementation can be deprecated and/or removed in subsequent PRs.

Closes #14687

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[codspeed-hq]

CodSpeed Performance Report

Merging #14694 will not alter performance

_{Comparing simplify-flow-parameter-extraction (140621b) with main (85bea69)}

Summary

✅ 5 untouched benchmarks

[bunchesofdonald]

Just the one question, assuming they get a reasonable error in the end this looks great.

[bunchesofdonald]

What happens if the flow uses the non-existent package? Like in this case, what if the flow was:

        from not_a_module import not_a_function
        from prefect import flow
        @flow(description="Says woof!")
        def dog():
            return not_a_function('dog')

Would the user still get a reasonable error message? This was the original reasoning for the placeholder flow variant, so that a user attempting to execute the flow in an environment where the package was unavailable would be told so.

[desertaxle]

In this case, it'd raise a NameError since not_a_funtion isn't defined in the namespace the flow was loaded in. Do you think that's reasonable? I could wrap the flow's __call__ to reraise the ScriptError, but I don't want to accidentally obfuscate exceptions from the flow's function.

[bunchesofdonald]

Name error is probably good enough for now, and we can revisit if it's confusing.

[bunchesofdonald]

I'm loving this, much cleaner.