API Key support for APM Integration when using non-Elasticsearch output

Define if/how APM Integration should support API Keys for auth handling between APM Agents and APM Server (`apm-server.auth.api_key.enabled: true`) when a different output than Elasticsearch is configured.

As pointed out by @joshdover in https://github.com/elastic/apm-server/issues/7025#issuecomment-1008792867 the output might be changed after the apm integration was added and configured.
For now, the API Key setup could either not be supported or a yaml box for ES host configuration could be added.

See https://github.com/elastic/apm-server/issues/7025 for more discussion for a similar issue.