Document "client_credentials" grant

[christiansmith]

Anvil Connect supports two-legged OAuth 2.0, where a client requests an access token for it's own use, rather than on behalf of a user. This feature is presently undocumented.

[simonrenoult]

:+1: We're trying to implement this right now but the CLI returns a Validation Error because the provided grant_types: ["client_credentials"] and response_types: ["token"] are said to be mismatched. Documentation would help!