proper error handling in authentication middleware

[JonasScholl]

When an error in an starlette AuthenticationBackend occurs, a AuthenticationError must be raised, other exceptions may produce errors like: 'RuntimeError: Caught handled exception, but response already started.' (see starlette documentation)

This PR:

• catches all exceptions that occur in the verify_authorization_header callback and convert them into an AuthenticationError
• adds an optional error handler callback for specifically catching auth errors and returning a custom response (since this is already offered by the AuthenticationBackend implentation from starlette)
• does some type hint improvements, I couldn't resist 😂

[codecov]

Codecov Report

Merging #2 (7a6893a) into main (db8521f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main        #2   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           30        34    +4     
=========================================
+ Hits            30        34    +4     

Impacted Files	Coverage Δ
fastapi_auth_middleware/middleware.py	100.00% <100.00%> (ø)

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[yannicschroeer]

Nice 👌

[yannicschroeer]

Do you think this is explicit enough? 😄