Add labels and submit requirement for copybara

For now the submit requirement is never applicable.

Bug: b/403589436
Change-Id: Id375ed69b1c98e8f81f69a468bb17bb67f57714d
Reviewed-on: https://pigweed-review.googlesource.com/c/Pigweed-Base/+/280892
Reviewed-by: Oliver Newman <olivernewman@google.com>