Import mapping definitions from the "Agent Common Schema"

[jsoriano]

This is blocked by the definition of elastic/package-spec#441

Allow to import mappings from the "Agent Common Schema" into packages on build time.

This feature would work similarly to the import of external fields, but would import sets of fields from the schemas defined in elastic/package-spec#441.

I would propose to:

• Import these definitions into the current fields directories, in new files, so current versions of Fleet would benefit of definitions included in packages following this approach.
• It should be also a well-known name, so Fleet could eventually take over this behaviour if (TBD issue) is implemented.

So for example the mappings for add_host_metadata could be imported in fields/imported_beats_add_host_metadata.yml, and eventually Fleet can know if it should include these fields or not depending on the used policy.