refactor(oidc): Move qrcode module inside oidc

[zecakeh]

Because it uses the OIDC API and it is accessed via Oidc::login_with_qr_code().

The second commit makes LoginWithQrCode use OidcBackend instead of its own OauthClient. It will allow to share code when the server backend is switched to the oauth2 crate too. It will also allow to expose the device authorization grant directly in Oidc, if necessary.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 8 lines in your changes missing coverage. Please review.

Project coverage is 85.87%. Comparing base (2eb2ae7) to head (1b3f35a).

Files with missing lines	Patch %	Lines
...matrix-sdk/src/authentication/oidc/qrcode/login.rs	76.47%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4687      +/-   ##
==========================================
- Coverage   85.89%   85.87%   -0.02%     
==========================================
  Files         292      291       -1     
  Lines       33912    33930      +18     
==========================================
+ Hits        29129    29139      +10     
- Misses       4783     4791       +8     

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