Update registry rest client to support bearer authentication

[gtato]

This change allows schema registry clients to specify a token in case the registry server supports token/oidc authentication.

[cla-assistant[bot]]

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[iamed2]

When using Bearer auth from curl, we needed to provide additional headers for Confluent-Identity-Pool-Id and target-sr-cluster. Have you succeeded in using this branch to connect?

Additionally, needing to recreate the client whenever a token refresh is needed is cumbersome. Inserting a user-provided token-fetching callback as part of a custom auth for the Requests Session may be a more user-friendly option. A custom auth could look something like:

class BearerAuth(requests.auth.AuthBase):
    def __init__(self, token_cb):
        self.token_cb = token_cb

    def __call__(self, request):
        """Called directly by requests to add the Authorization header"""
        token = token_cb()
        request.headers["Authorization"] = f"Bearer {token}"
        return request

and you can hook this in by setting self.session.auth = BearerAuth(token_cb).