Kyle Hekhuis

Adds a helper method for validating client authentication and use it in `AuthorizationCodeGrant`, `DeviceCodeGrant`, `RefreshTokenGrant`, and `ResourceOwnerPasswordCredentialsGrant`. Putting the helper method in `DeviceCodeGrant` also fixes an issue where the client...

**Describe the bug** The `DeviceCodeGrant.validate_token_request` method always tries to authenticate the client ``` if not self.request_validator.authenticate_client(request): raise rfc6749_errors.InvalidClientError(request=request) elif not hasattr(request.client, "client_id"): raise NotImplementedError( "Authenticate client must set the "...