adds support for auth proxy header

[eyJhb]

Useful if you have authentication in front of wger, and want to use that instead of wgers authentication/signup methods.

Proposed Changes

Added a new setting, which can be used to setup proxy auth header, e.g. using Authelia to authenticate + create users on the instance.

Fixes #1847 , and potential VERY SIMPLE solution for #1805 ( #1797 )

Please check that the PR fulfills these requirements

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Added yourself to AUTHORS.rst

Other questions

• Do users need to run some commmands in their local instances due to this PR (e.g. database migration)? No

Remaining work

• [ ] Add to documentation.
• [ ] Flutter needs to support setting API key, instead of username/password.

[eyJhb]

Maybe this should only be checked for / like in Miniflux here https://github.com/1pav/miniflux/commit/5a4009625610f6896b3d5421cb81cb2d51d06120 , but I'm unsure what would be best tbh.

[rolandgeider]

BTW I haven't forgotte about this PR, but since it would add a new auth method I'd like to make sure we do it right. I've added this to the 2.4 backlog (so after the flexible routines get merged)