pkg_deb: support md5sums file

[adam-azarchs]

The md5sums control file is optional, but recommended and missing it will result in a lintian error. There are also some tools like older versions of aptly which required it. It's also handy for efficiently comparing the content of two packages without fully extracting them, e.g.

diff <(dpkg-deb -I pkg_1_all.deb md5sums) <(dpkg-deb -I pkg_2_all.deb md5sums)

Yes, I know it's 2025 and we shouldn't be using md5 for anything any more (it's not just insecure, it's also slow compared to better hashing functions). But that's what's in the standard.

I'm not proposing that rules_pkg should necessarily be responsible for creating the file, but it should be allowable to supply one to the pkg_deb rule to support cases where one is desired or required.