Add a release workflow.

[aiuto]

Add a release workflow to:

• create tag
• create distro
• build release notes
• create github release in draft form
  • Hand tweak the generated notes. I like to pull new features up to the top.

Test plan:

• Run it from with gh workflow run
• https://github.com/bazelbuild/rules_pkg/releases
• verify that the asset it builds and uploads to the release page is identical to the one built in the past.

Next steps

• Create a BCR workflow to run manually after individual releases are integration tested.

[cgrindel]

Added a suggestion for a missing word. Otherwise, LGTM.

[aiuto]

I just verified that I could point to the 1.2.0 release that was built with this - and that my project kept working.
So, I'm going to merge..