refactor(oauth): Allow to use any registration method with any login function

[zecakeh]

This creates a ClientRegistrationMethod enum and use it for OAuth::login and OAuth::login_with_qr_code, allowing notable to use the OAuthRegistrationStore every time.

We can get rid of url_for_oidc since everything can be done with OAuth::login now, and we also merge OAuth::finish_login and OAuth::login_with_oidc_callback() to use the simplest API.

Finally there are some optimizations to reduce the number of requests by reusing the AuthorizationServerMetadata.

This can be reviewed commit by commit.

[codecov]

Codecov Report

Attention: Patch coverage is 95.06173% with 4 lines in your changes missing coverage. Please review.

Project coverage is 86.50%. Comparing base (22cbce8) to head (c6d0bae).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/authentication/oauth/mod.rs	94.44%	3 Missing ⚠️
...atrix-sdk/src/authentication/oauth/qrcode/login.rs	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4827      +/-   ##
==========================================
+ Coverage   86.47%   86.50%   +0.02%     
==========================================
  Files         297      297              
  Lines       34584    34600      +16     
==========================================
+ Hits        29907    29930      +23     
+ Misses       4677     4670       -7     

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[Hywan]

More clean ups 👏! More clean ups 👏!

Thanks 🙇.