Validate Gem dependencies before release

[ahayworth]

This PR adds a way to check that the gems we are releasing will not be broken upon said release.

We run this check as a GitHub action that only fires on pushes to release/** branches, and we accomplish it by:

• Getting a list of changed gemspecs in this PR (ie: the things we will be releasing)
• Loading all gemspecs in the repo
• Loading the latest released version of every gem in the repo, directly from rubygems.org
• For each gemspec in the repo, we then check the dependencies:
  • If the dependency is included in this release request, we check that the dependency will be satisifed by interrogating the loaded gemspecs directly.
  • If the dependency is not included in this release request, we then construct a fake gemspec with the info on rubygems.org, and then see if it's a match.

We'll fail if there is a gem dependency that doesn't satisfy the version constraints based on what we're about to release or what's already available on rubygems.org - or if it's not published at all.

It's kinda hard to test this locally, so I'm not 100% if the changed_gems logic with FETCH_HEAD works correctly.