[Ingest Manager] Share CONFIG_CHANGE actions between agents

[nchaulet]

Description

Currently we are creating one action (saved object AgentAction) for every agent, during checkin, this is really ineficient, and create a lot of non necessarily CPU usage and ES docs.

We could move to a model where we have an action per config change, and all the agents share this action, and replace the values that are per agent at runtime in the checkin handler (output api key).

Also this action could be generated when we create a new revision of agent config.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[ph]

@nchaulet going to assign this to you, this look linked to the performance efforts.