[Fleet] Support configuring and referencing multiple outputs

[simitt]

Describe the feature:
Let users define other outputs than the one global output and reference them for subprocesses. At the moment only one global output can be configured. This forbids to e.g. send monitoring data to a dedicated monitoring cluster.

Describe a specific use case for the feature:
Elastic Agent runs a monitoring metricbeat and filebeat that are observing the agent itself and its subprocesses. These monitoring beats should ship data to a dedicated monitoring cluster, while the other processes, for example apm-server should ship to a production cluster.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[chrisronline]

+1!

[nugroho-exp]

We have similar requirement to have a different Elasticsearch URL for some policies (in our case to use Private link when available). It would be nice if we can override Elasticsearch URL at policy level.

Discussion link: https://discuss.elastic.co/t/fleet-override-elasticsearch-url-at-policy-level/275982

[jen-huang]

Closing in favor of #104983.