Automatically support "Add Projects" in Herald for all relevant objects

Summary: Ref T7849. If the adapted object implements PhabricatorProjectInterface, support the ADD_PROJECTS action.

Test Plan:

• Wrote a Differential rule with "Add Projects".
• Updated a revision.
• Got projects added.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7849

Differential Revision: https://secure.phabricator.com/D12504