[eem] expose getServices API

[klacabane]

Summary

We should hide the querying logic to get services from .entities* indices behind an API so consumers don't have to dig into the implementation. #184548 adds a public client that already proxies few queries to the entity manager, we can implement getServices there.

Note that this logic is currently implemented in the apm plugin (see #183605), but eventually ownership belongs to entityManager plugin

AC

• entityManager plugin exposes GET /internal/api/entities/services endpoint that accepts query options (to be defined)
• entityClient exposes getServices methods that queries the new endpoint

[elasticmachine]

Pinging @elastic/obs-knowledge-team (Team:obs-knowledge)

[tommyers-elastic]

possible dup of #182850