Refactor Proxy outpost OAuth logic

The current outpost OAuth logic relies on the duration of the access token, leaving that as the only way to configure session duration. The token is never refreshed which is also not ideal. ~~This also causes proxy sessions to remain after a user logs out of authentik as the token isn't rechecked.~~

Changes required:
- Relatively short access tokens (10 minutes?) (potentially not exposed via API)
- Configure duration of refresh token in proxy provider
- Refresh access token when it's close to expiring/has expired in outpost
- Update session and refresh session expiration when access token has been refreshed