Support configuring APM modes

[mshustov]

With #102624 Kibana will be able to implement three different modes:

mode	kibana.yml	finalized agent config
APM agent sends data to the APM server, Kibana relies on the agent to implement log correlation.	active: true	{ active: true, disableSend: false }
APM agent doesn't send data to the APM server, but Kibana uses it to implement log correlation.	active: undefined	{ active: true, disableSend: true }
APM agent is disabled completely.	active: false	{ active: false, disableSend: true }

Kibana should provide sensible APM agent defaults for every mode.
A Kibana user should be able to switch between modes.
New Kibana default shouldn't lead to a performance regression (verify with load testing).

blocked by #102624

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[afharo]

should we consider a 4th option? [1] but sending to an Elastic-owned server (if users want to help us with performance metrics). What do you think?

[mshustov]

but sending to an Elastic-owned server

It's [1] supplied with a special URL. The URL should be provided via the config file or an env var, not hardcoded in the Kibana sources. So I don't believe it requires an additional mode.

[trentm]

[email protected] is now published with the contextPropagationOnly config var.