Gitea upstream oauth2 support

[esticle]

Is your feature request related to a problem? Please describe.
Want to try out new Element X with my existing OIDC provider Gitea

Describe the solution you'd like

From the documentation it is unclear which part of the oath2/Github portion of the config should be considered as part of such a configuration, I have cobbled this together thusfar:

upstream_oauth2:
  providers:
    - id: "01HFS67GJ145HCM9ZASYS9DC3J" # Github ID
      human_name: Gitea
      brand_name: gitea
      discovery_mode: disabled
      fetch_userinfo: true
      token_endpoint_auth_method: "client_secret_post"
      client_id: "da6g28c0-xxxx-xxxx-xxxx-ac3208645d42" # TO BE FILLED
      client_secret: "xxx" 
      authorization_endpoint: "https://gitea.mydomain/login/oauth/authorize"
      token_endpoint: "https://gitea.mydomain/login/oauth/access_token"
      userinfo_endpoint: "https://gitea.mydomaint/api/v1/user"
      scope: "read:user"
      claims_imports:
        subject:
          template: "{{ id }}"
        displayname:
          action: suggest
          template: "{{`{{ user.full_name }}"
        localpart:
          action: ignore
        account_name:
          template: "@{{ user.login }}"

On the Gitea site I've made the callback URL https://mas.mydomain/upstream/callback/01HFS67GJ145HCM9ZASYS9DC3J

However it's not too happy (invalid request).

Describe alternatives you've considered

Somewhat of a OATH/SSO noob so please correct me if I'm wrong and said feature request becomes a support request.

Thanks!
E