APM Central Management using external data sources (e.g. Consul)

**Is your feature request related to a problem? Please describe.**

In some cases, configuration cannot be stored in Elasticsearch and/or it is not possible to use Kibana APM to manage the configuration.

**Describe the solution you'd like**

Ability to pull/search dynamic configuration from an external system (e.g. Consul).

Given the fact there are a lot of possible external systems/databases, maybe an approach might be to improve the documentation about the configuration retrieval API in APM Server (https://www.elastic.co/guide/en/apm/server/current/agent-configuration-api.html) and correct handling of `etags` for proper caching on APM Server side.

**Describe alternatives you've considered**

None at the moment.