Add CI to ensure all `CODEOWNERS` are assignable

description

Currently not all CODEOWNERS are assignable - which means that when relevant PRs come up we are unable to assign reviews to the relevant people

This issue is being addressed manually but it would be useful to have CI to ensure that this is kept up to date

One implication here is that a senior maintainer will have to sign off (eg by adding assignable to the relevant dev) any new contribs that have new devs listed as codeowners