Add client credential auth method

[dirkkul]

This adds a new oidc authentication method to the client and refactors the authentication handling

You can use this authentication mode with

client = weaviate.Client(
        url=URL, auth_client_secret=weaviate.AuthClientCredentials(client_secret=CLIENT_SECRET, scope=SCOPE),
    )

Tested with okta and azure

[StefanBogdan]

Do we want to add same test for Google authentication?

[dirkkul]

Do we want to add same test for Google authentication

Google sadly doesn't support client credentials. I haven't tested user/pw, but I'll make sure that the next method is supported by google