Add logfire.add_non_user_code_prefix function for library developers

[dmontagu]

Adds a public API for adding items to the tuple of non-user-code-prefixes, so that the frames are excluded in logfire spans. This will be used in pydantic_ai; see occurrences of the following in that codebase:

# while waiting for https://github.com/pydantic/logfire/issues/745
try:
    import logfire._internal.stack_info
except ImportError:
    pass
else:
    from pathlib import Path

    logfire._internal.stack_info.NON_USER_CODE_PREFIXES += (str(Path(__file__).parent.absolute()),)

Closes #745

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[alexmojaki]

Please add it to logfire.testing since that's what this is really intended for and the logfire namespace is already crowded, and make it accept a module for usability.

[alexmojaki]

I've decided against the logfire.testing idea because that module requires pytest. I think the module thing would be good but I guess it's not essential and I'd like to merge this.