Switch authentication from current OAuth to use Keycloak

[autumnfound]

Within the Eclipse Foundation, we are in the process of slowly deprecating the usage of the OAuth client provided through the Drupal website and are switching to Keycloak for OIDC services. We will want to change over to use Keycloak for this service, and align with the upstream API change to consume the new authentication source.

For more info, see https://gitlab.eclipse.org/eclipsefdn/it/api/eclipse-openvsx-api/-/issues/38