Import mapping definitions from the "Agent Common Schema"

_This is blocked by the definition of https://github.com/elastic/package-spec/issues/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 https://github.com/elastic/package-spec/issues/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.

Start by reading package-spec issue 441, then inspect the existing `external` field import behavior in the build process. The work is done when Agent Common Schema definitions can be imported into package fields directories using a well-known filename convention, such as `fields/imported_beats_add_host_metadata.yml`, while preserving definitions for current Fleet versions.

Written by the indexing model from the issue text.