Add ability to add headers alongside PAT

[gitnew12]

First, I want to say THANK YOU for making this app! It has made me use Firefly much more often, and I can’t thank you enough.

Currently, I am using Authentik and have Unauthenticated Paths set to ^/api/.* so that Abacus can connect to Firefly’s API. A more secure approach would be to allow Abacus to pass headers to Authentik, such as a Basic Authorization header. This way, you can create a Service User specifically for Abacus in Authentik, with access only to Firefly. Then generate an Application Token in Authentik for the service user, and include the username and token (Base64 encoded) in the header. Authentik would then authorize Abacus (service user) to access Firefly.

I use the method described above for Ruddarr (iOS app) to connect to Sonarr/Radarr and Tautulli Remote (iOS app) to connect to Tautulli.

[victorbalssa]

I do not have time to investigate this edge case right now, but feel free to dig into it and submit a PR. I will do my best to review, merge, and deploy it.