feat(actions): Deputy scanner

[dnys1]

• Creates a new package (aft_common) which includes non-CLI logic which can be shared with any Dart package (e.g. pub_server and actions)
• Vendors the subset of the git package we need and makes it cross-target compatible (works with dart:io and Node)
• Improves constraint update logic
• Adds custom Dependabot (Deputy) and introduces a new GitHub action which checks for latest dependencies and creates PRs for outdated deps with the correct constraints and in all applicable packages