Working label checker in Go

[johnboyes]

Written in Go, using Mage: https://magefile.org/

Code is loosely based on this example:
https://www.sethvargo.com/writing-github-actions-in-go/

Checks for exactly one of the specified labels, and fails if there are
none or more than one.

Future enhanced functionality could be to allow the client application
to also specify a set of labels none of which should be present, and
a set all of which should be present.

Two noteworthy things to improve in future commits:

1. There is a bug whereby a PR can show that the check has passed, but
   still not allow the pull request to be merged. This is because GitHub
   Actions create a separate new instance of the check for some different
   event types, e.g. the labeled event creates a new check instance
   separate from the push event checks. So if we, for instance:

2. push a commit when there is no required label

3. add the required label
   then the lableled instance of the check will pass, but the push will
   still show the initial failure.
   The proposed fix for this (in a future commit) is to retrieve the GitHub
   labels for the pull request via the GitHub API, instead of using the
   labels in the GitHub Action's event JSON (which is a frozen snapshot of
   the labels at the time of committing and therefore does not stay up to
   date with subsequent label changes). Then if we rerun the failed push
   job after amending the labels, the check will pass.

4. The github.CheckLabels() method should return a boolean (along with
   a message) indicating whether the labels are valid or not for the pull
   request, rather than returning an error. As per:
   https://martinfowler.com/articles/replaceThrowWithNotification.html
   If this method returns false, we'll then return an error in the Mage
   command (which will ensure a non-zero exit code), so that the GitHub
   Action fails the check.