Closes #8407

Changes proposed in this pull request:

• This PR sets OIDC to allow FORWARDED-FOR headers and use BASE_URL to fix a scenario in which OIDC can't use https callback URLs behind a reverse proxy

How to test the feature manually:

1. Setup FreshRSS with docker
2. Setup a Reverse Proxy (eg. Traefik) to serve FreshRSS over https
3. Setup an OIDC provider (eg. PocketID)
4. Set FreshRSS to use OIDC
5. You will notice that despite being served over https FreshRSS will insist in using OIDC over http

Pull request checklist:

• clear commit messages
• code manually tested
• unit tests written (optional if too hard)
• documentation updated

Additional information can be found in the documentation.