Add explicit support for User/PW, client credentials and Bearer token oidc auth flows

[dirkkul]

… oidc auth flows

Before you could use all (most?) oauth methods by creating a oauth2.NewClient as described in the docs. However, creating this client isn't straight forward and you need to supply configuration values that we can get automatically from the weaviate oidc configuration.
(Before looking into the code I wasn't aware that this includes authentication methods other than supplying a token).

This PR adds a new NewConfig function, that creates an oauth2 client as part of the client configuration:

NewConfig(Host string, Scheme string, AuthConfig auth.Config, Headers map[string]string)

The possibilities for auth.Config are:

• auth.BearerToken{AccessToken:*token*, ExpiresIn: *seconds*, RefreshToken:*token*}
• auth.ResourceOwnerPasswordFlow{Username: *user*, Password: *pw*, scope:*scopes*}
• auth.ClientCredentials{ClientSecret: *secret*, scopes: *scopes*}

To get a client with authentication enabled, use:

clientCredentialConf := auth.ResourceOwnerPasswordFlow{Username: "*user*", Password: *pw*}
cfg, err := weaviate.NewConfig("localhost:"+fmt.Sprint(WCSPort), "http", clientCredentialConf, nil)
client := weaviate.New(*cfg)

and without:

cfg, err := weaviate.NewConfig("localhost:"+fmt.Sprint(WCSPort), "http", nil, nil)
client := weaviate.New(*cfg)

There are tests that test authenticating with a local weaviate and WCS, Okta and Azure as authentication providers.

[antas-marcin]

I would suggest to refactor this goroutine to use channels with ticker instead of sleep. it would make also a code a little bit more readable

[dirkkul]

refactored the goroutine a bit, please have another look