Update KeyClient and associated classes to match API key response

[michaeljolley]

The Key class and KeyClient are not up to date with the /projects/{project_id}/keys endpoints of the Deepgram API. This will cause errors or invalid responses.

Documentation for the endpoint can be found at https://developers.deepgram.com/api-reference/#keys

This like would change the responses of the ListKeysAsync, GetKeyAsync, and CreateKeyAsync methods.