Allow auth property of httpx to be passed to schema registry client

[ericdevries]

Description

We have some custom logic to acquire oauth tokens that need to be sent as a header to the schema registry. Unfortunately, the schema registry client only allows for basic auth and ssl auth. We worked around it like this:

class SchemaRegistryClient(ConfluentKafkaSchemaRegistryClient):
    def __init__(self, config: SchemaRegistryConfig):
        config_dict = {
            "url": config.url
        }
        super().__init__(config_dict)

        if config.use_oauth:
            # override httpx client because we cannot configure a custom auth class
            self._rest_client.session = httpx.Client(
                verify=self._rest_client.verify,
                cert=self._rest_client.cert,
                # This is the property that can't be passed along
                auth=AADAuth(OauthClient(scope=urlsplit(config.url).netloc)),
                proxy=self._rest_client.proxy,
                timeout=self._rest_client.timeout,
            )


class AADAuth(httpx.Auth):
    def __init__(self, oauth_client: OauthClient) -> None:
        self._oauth_client = oauth_client

    def auth_flow(self, request: httpx.Request) -> Generator[httpx.Request, httpx.Response, None]:
        token = self._oauth_client.get_token()
        request.headers["Authorization"] = f"Bearer {token}"

        yield request

Allowing the auth property to be passed to the schema registry client would make it a lot more flexible for anything outside of basic auth and certificate auth.

How to reproduce

Checklist

Please provide the following information:

• confluent-kafka-python and librdkafka version (confluent_kafka.version() and confluent_kafka.libversion()):
• Apache Kafka broker version:
• Client configuration: {...}
• Operating system:
• Provide client logs (with 'debug': '..' as necessary)
• Provide broker log excerpts
• Critical issue

[Claimundefine]

https://github.com/confluentinc/confluent-kafka-javascript/blob/master/schemaregistry/rest-service.ts#L27C1-L36C2

Hello, we are looking to implement OAuth authentication by end of Q1, similar to what we have in our Javascript client. It would allow setting a static Bearer token or allow users to input their client credentials, where the client would handle retrieving, caching, and refreshing the Bearer token from your OAuth Identity Provider.

Would this be the functionality you are looking for?