CRUD for OIDC configuration of Proxy

We need to have the same behaviour in the UI as in the API.
We need a CRUD API for OIDC configuration.

The OIDC configuration belongs to a Proxy, so the endpoint would be /admin/api/services/[service_id]/proxy/oidc_configuration.xml|json

We did not choose to add it to the proxy endpoint /admin/api/services/[service_id]/proxy as later we want to be able to configure the OIDC configuration per application.

Note: Currently we do not update every application when changing the service OIDC config. When application is created or updated the configuration is propagated.