[al clone] Expose FixtureFunctionDefinition as part of the public API

[jacksonsr451]

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• Closes Expose FixtureFunctionDefinition as part of the public API #14853

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Summary

Exposes FixtureFunctionDefinition as pytest.FixtureFunctionDefinition and adds it to pytest.__all__.

Adds a regression test verifying that objects created by @pytest.fixture are instances of the publicly exposed pytest.FixtureFunctionDefinition.

[bluetech]

There is already a PR open for this.