Support required vs. optional acks

In support of future webhook work and direct use of CODEOWNERS, add a new flag to each subsystem titled 'required-approval', and default it to false. Subsystems will have to opt in to being a required approval for merge requests that touch "their" code, instead of always being required.

This should help reduce the required approval volume considerably, and the required review load from a number of people.

Note: the mm and net subsystems, along with all maintainers entries, are being set as required-approval: true, because we know these groups have already expressed their desire to be required approvers for their code areas.

Signed-off-by: Jarod Wilson jarod@redhat.com