Creates a new access token (grant_type=client_credentials) or refreshes an existing one (grant_type=refresh_token). Request must be application/x-www-form-urlencoded.

Create token (client_credentials): Requires username and optionally scope, expires_in, refreshable. Supported scopes are admin and groups. Non-admin users can only create tokens for themselves and cannot request admin scope.

Refresh token (refresh_token): Requires grant_type, refresh_token, and access_token; optionally expires_in and refreshable.