Proxy authentication flow

[lufixSch]

This PR aims to add forward auth/proxy auth to this project as discussed in #461. In forward auth a proxy routes requests through a third party authentication service and only if this service returns a 200 response, the request is passed to the actual service. The authentication service will add one or many authentication headers to the request depending on the service which can be used to identify the authenticated user.

Changes made

In an ideal implementation the user would be authenticated automatically (e.g. no 'login' button has to be pressed) but with the current implementation I wasn't able to think of a viable solution which doesn't require a DB lookup on every API request. Instead, I added another provider ("proxy") and a corresponding login button in the frontend. When this button is pressed an API request to /auth/proxy is made which checks for the authentication header and returns an authentication token if the header is valid.

The current implementation is a minimal proof of concept in order to see if it could work. It's the first time I worked with GO, and I'm open to feedback.

Missing features

• [x] Settings for enabling proxy auth and specifying the name of the authentication header
• [x] Allow creation of a new user if it doesn't already exist
• [x] Protect /auth/proxy route so that it only works if proxy auth is specifically enabled

[lufixSch]

I added automatic user creation. The current behavior is the following: If the user already exists either with no UserType (e.g. username + password) or PROXY_USER they are authenticated. If no user exists with that username and either no UserType or PROXY_USER as type then a new user with UserType PROXY_USER is created. A user of type PROXY_USER can not log in with username and password.

[lufixSch]

The authentication header can now be specified using the watcharr.json config file or through the admin settings in the frontend

The /auth/proxy route will return 403 as long as no authentication header is specified in the config. It also returns 403 if the authentication header is missing or if it is empty (e.g. "")