OTel: add the custom services REST endpoint family

A custom service turns an OpenTelemetry metric into a monitored service on a host. This adds the endpoint family under which the upcoming REST endpoints (create, list, update, delete) are documented, registered in the ultimate, ultimatemt and cloud editions.

CMK-38039

Change-Id: Ifb5a30fa567db437f61aff13c565e12197d37868