Extract PathOwners from StoredValue.

Can be reused by other plugins.
Make groupId more specific.
Introduce gerrit-owners-parent.

This is in preparation for creating an auto assigning plugin that uses
the owners config.
Add auto assign plugin.

Depends on gerrit-owners for parsing the OWNERS files and then will
automatically set the reviewers whenever a change is created or updated.

It will only add new reviewers, never removing old ones.