Improve typing in Fab

[jscheffl]

While I was hunting for a bug in OAuth I realized that the typing of Python code in Fab is... missing in most part. This PR adds more typing hints to the providers/fab/src/airflow/providers/fab/auth_manager/security_manager/override.py - and as a consequence I found a lot of warnings and mypy errors alongside which I needed to fix to make tests and mypy happy again.

Not sure whether my type fixes are introducing some kind of breaking changes... but I assume Fab is "not a kind of public API"? Therefore would merge only based on an expert review like @vincbeck or @potiuk

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[potiuk]

Cool

[vincbeck]

Nice! Thank you :)