Add extend-project tag to support adding groups to an existing project

Currently, if a local manifest wants to add groups to an existing
project, it must use remove-project and then re-add the project with
the new groups.  This makes the local manifest more fragile, requiring
updates to the local manifest if the original manifest changes.

Add a new extend-project tag, which supports adding groups to an
existing project.

Change-Id: Ib4d1352efd722a65dd263d02644b9ea5ab6ed400
Merge "Add extend-project tag to support adding groups to an existing project"