Agent configuration should indicate whether it was applied remotely or not.

This entails removing the logic around Etag and serialization of Kibana responses. Instead, Kibana will calculate Etags and apm-server will pass it down to the agents. apm-server will also forward the etag sent from the agents to Kibana, so it can know if the agent is up to date or not.

Kibana ticket: elastic/kibana#43354