Introduce an x-pack-apm plugin in Elasticsearch, which installs index templates, component templates, and ingest pipelines for Elastic APM.

This removes the Fleet integration dependency for the apm-server binary, and generally reduces the Fleet focus to central configuration and fleet management, where desired.
This will also simplify APM Server by removing the possibility of index templates being missing on startup.

@axw created a PoC, which should be used as a basis. Some points, such as automatic rollover need more clarification.