Evaluate using CodeOwners to control commit access

Currently Apple-external developers are made into GitHub “Collaborators” to allow issues and PRs to be assigned to them. This does not permit them to modify issues, such as assigning themself to an issue that they filed, or modifying the labels or milestones of issues or PRs.

GitHub Code Owners was pointed out as a potential avenue for allowing contributors to be granted the full “contributor” status that allows them to do the above modifications, but without granting the ability to merge PRs or directly push commits. This should be tested and evaluated to see if that is the case, and implemented if so.

Start by testing GitHub Code Owners in the FoundationDB repository and compare the resulting permissions for issue and PR assignment, label and milestone changes, merging, and direct pushes. Done means documenting whether Code Owners provides the required contributor access without merge or push access, and recording an implementation decision.

Written by the indexing model from the issue text.