get_credential() in CredentialProvider is not async for async client

[yulin-li]

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The async version of Redis client takes credential_provider as argument while the get_credential() method is not an async method.

I think a better version should be

class CredentialProvider:
    """
    Credentials Provider.
    """

    async def get_credentials(self) -> Union[Tuple[str], Tuple[str, str]]:
        raise NotImplementedError("get_credentials must be implemented")