manifest: add support for path variables

[mtojek]

Some configuration files may use path variables:

    os.darwin:
      default:
      - /usr/local/var/mysql/{{.builtin.hostname}}.{{.builtin.domain}}.err*

The only workaround at the moment is to set a sample value or skip the entire line.

[ruflin]

This depends on elastic/beats#17340

[ruflin]

Dynamic inputs should solve this issue: elastic/beats#19225

[ruflin]

Closing this issue as I think it is not a problem of the package-registry anymore to solve this and potentially not even of the package-spec as it is directly part of the input definition. Will follow up in the right repo if needed.