Add a Fleet / Elastic Agent extension

[antoineco]

Elastic Agent takes the concept of monitoring agent further than Metricbeat, Filebeat and Heartbeat. Not only it embeds these, but each agent can (optionally) join a Fleet and receive its configuration from a central place, which under the hood is managed via Kibana and backed by Elasticsearch.

I believe it could be useful for users to have an extension they can include in their stack and play with Elastic Agent (test third-party integrations, visualise monitoring data in the dashboards that ship with Kibana, etc.).

Overview: https://www.elastic.co/guide/en/fleet/current/fleet-overview.html

[antoineco]

Additionally, we should remove the APM Server extension, since the standalone APM Server is deprecated (now part of Elastic Agent).