Add overloads for URL parameter functions authlib.common.urls

[mrhx01]

params_to_qs:

Reason:

1. The implementation accepts dict[str, str] (via params.items()), so the type hint should reflect this.

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extract_params:

Reason:

1. The function accepts None and returns None
2. str input (query string) is also supported and may return None for invalid input

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[mrhx01]

Thanks for the review! I've applied both suggestions:

1. Changed add_params_to_qs to use union instead of overload
2. Added tuple[tuple[str, str], ...] to extract_params overload

[srittau]

Thanks!