Automate adding collaborators based on PR modified changes

[sethmlarson]

Currently GHSA doesn't automatically add CODEOWNERS to GHSAs when that code is modified. This bot can do this for us automatically by:

• When a GHSA PR is modified
• Run the changed filenames against the CPython CODEOWNERS for the main branch
• If there are matches, add those users to the collaborating_users / collaborating_teams field in the GHSA

[encukou]

I'm not sure we should open embargoed issues to CODEOWNERS.
The mechanism can be used just to get GitHub notifications about a given file; it doesn't necessarily imply actual “ownership”.

Maybe we should instead assign existing PSRT members according to CODEOWNERS? (and get more people on PSRT of course)

[sethmlarson]

@encukou @hugovk Maybe this needs some clarification about what it means to be in the CODEOWNERs, because my read is that these are the folks we'd be adding manually to a GHSA collaborators in case we were looking for reviews (akin to the Experts Index with an opt-in to being assigned to issues).