refactor: generalize SSO auth

[motorina0]

closes #2212

Summary

See issue #2212 . Prepare to allow more SSO providers (eg: Authentik, Keycloak or Authelia.)

The /auth endpoint now does not explicitly uses google or github. Any number of SSO Providers can be used now.

In order to add a new provider one must:

1. Implement the ProviderSSO class (eg: KeycloakSSO)

• this class must be added in the lnbits.core.sso.provider. The full namespace is lnbits.core.sso.provider.ProviderSSO
• several implementation examples can be found here: https://github.com/tomasvotava/fastapi-sso/tree/master/fastapi_sso/sso
• ideally the new implementation is accepted by fastapi-sso. Please also open a PR to the above repo (fastapi-sso) with the new implementation.

2. Create a settings class for the new Provider (eg: KeycloakAuthSettings). See GitHubAuthSettings as an example. Then add this class to EditableSettings.

• don't forget to update the env.example file

3. Add the new provider to AuthMethods enum
4. Update the Login UI with the new auth provider

[codecov]

Codecov Report

Attention: 38 lines in your changes are missing coverage. Please review.

Comparison is base (98338ff) 59.26% compared to head (f171004) 59.29%.
Report is 3 commits behind head on dev.

Files	Patch %	Lines
lnbits/core/views/auth_api.py	19.14%	38 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2263      +/-   ##
==========================================
+ Coverage   59.26%   59.29%   +0.03%     
==========================================
  Files          60       60              
  Lines        9026     9036      +10     
==========================================
+ Hits         5349     5358       +9     
- Misses       3677     3678       +1     

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[prusnak]

utACK