GW refactoring: Login service

[olevski]

This should replace the service that is currently implemented in the gateway in python. This service does the following:

• intercepts the authorization code from the oauth flow to get access and refresh tokens
• stores the refresh and access tokens in redis
• creates the user session in redis (upon successful login for registered users)
• creates the user session in redis for anonymous users (this requires no login)
• guides a registered user through all the services (i.e. keycloak and gitlab) that they need to log in and redirects to the proper url(s)
• logs the user out and removes their session
• provides the user information about their session - i.e. is the session valid and when it expires
• provides the ability to refresh a session - for this we issue a new session ID

The idea is to start with a api spec here, generate code from it with this library and then implement the code to handle all the endpoints from the spec.

The idea is that everything follows the api (i.e. swagger) spec. So for adding endpoints we do the following:

1. Edit the api spec
2. Generate code
3. Implement the endpoint

[olevski]

Use issue #622 to finish the api spec here before you start this issue.

[olevski]

The initial implementation of this used components from the oauth2 proxy library we use in every session. However this proved to be very messy. So a rewrite with https://github.com/zitadel/oidc was initiated but never finalized.