Improve authentication mechanism

[AMontagu]

Today DSG authentication mechanisme work with custom metadata to make it compatible with Django and DRF JWT token authentication. We are not sure for now if this specific behavior is fully compatible with the Token-based authentication with Google described here: https://grpc.io/docs/guides/auth/

Follow example to test behavior :

• https://github.com/grpc/grpc/blob/master/examples/python/auth/async_customized_auth_server.py
• https://github.com/grpc/grpc/tree/master/examples/python/auth#authentication-extension-example-in-grpc-python