Add release/version automation using release-please

[petejohanson]

See #2513 for some discussion here.

Automate our versioning/release process leveraging the https://github.com/googleapis/release-please-action

Release major.minor.0 version from main branch:

• Update the new app/VERSION to match
• Create-or-update the v{major}.{minor} tag
• Create a v{major}.{minor}.0 git tag
• Create GH release.
• update unified template's config/west.yml to point to new version branch

Release a new {major}.{minor}.{patch} version from major.minor-branch branch where patch > 0:

• Update the new app/VERSION to match
• Update the v{major}.{minor} tag
• Create a v{major}.{minor}.{patch} tag
• create a GH release