ci: add job to verify binary size (#475)

This PR adds a job to diff binary sizes introduced in changes. As
implemented, the action runs `git merge-base` to find a common ancestor
with main, fetches a completed build from CI (it will try to up to 5
commits back in case CI hasn't completed for the commit returned by `git
merge-base`), and outputs a diff.

GitHub actions with a `pull_request` trigger are unable to comment on
PRs. As such, this implementation fails the check if the size difference
is greater than a threshold. In the case where we're ok with the size
increase, my understanding is that we can force the merge without the
check passing.

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Co-authored-by: Justus Camp <[email protected]>
Co-authored-by: John Starks <[email protected]>

Author: 3 people