policy: Cross Repo Commit Policy

Add Cross Repo Commit policy that was originally discussed in January
2023 by core, and ratified by the current core team on XXXX.

Sponsored by: Netflix
Reviewed by: ziaee, 0mp, kevans, glebius, adrian
Approved by: core (24-hour rule)
Differential Revision: https://reviews.freebsd.org/D57514