Add `common_repository` support to `maven`, `npm`, and `python` remote repository configurations in the `artifact-registry` module. This replaces the deprecated `custom_repository` feature which is now discouraged by the provider.
Existing README example `registry-mirror` has been updated to use `common_repository`. A legacy test case `legacy_custom_repo` has been added to the bottom of `README.md` to ensure backward compatibility for `custom_repository` continues to work.
TAG=agy
CONV=ffe77e65-ccef-4701-95e6-4ba2d2446f1b
48fdf03233