Add support for additional authentication methods

[j1mbl3s]

Avoid duplicated feature requests

• There are no open or closed feature requests that are related to this request

Description

I would like Plant-it to accept authentication from additional sources, such as forwarded proxy headers (X-Forwarded-User, X-Forwarded-Email, etc.), or maybe OAuth/OIDC but I know that is a larger effort and I'm not sure that whole flow is entirely necessary for this application.
This would allow Plant-it to offload the storage of authentication details to another service (it would still need to keep track of user data, though). SSO would be an option for environments that support it(!).

There would need to be a few additional configs for forwarded auth:

1. FORWARD_AUTH_ENABLED - true/false value which enables or disables the feature to accept forwarded authentication headers
2. FORWARD_AUTH_HEADER - string specifying the HTTP header used to pass the user id (i.e. X-Forwarded-User, X-Forwarded-Email, etc)
3. FORWARD_AUTH_TRUSTED_IP - a list of IPs to accept forwarded authentication from (i.e. from the IP of the incoming request); accept from any IP if it is empty/undefined

[acidRain-burns]

I would definetly like SSO via OIDC, a templated/generic method might notbe too hard. An example is Komga's implemenation: https://komga.org/docs/installation/oauth2/

I use the generic for my selfhosted PocketID, but many use Google/Microsoft/Apple/GitHub or other selfhosted like KeyCloak and Authelia. The templated method means you dont have to make one for all of these different providers. Hope this helps.

Love the project by the way <3