Add support for merge drivers when merging PRs ideally on a repo-by-repo bases

We should allow support of git merge drivers on a repo-by-repo basis for PRs and branch model auto merges. This could include built in merge drivers (such as union) and custom merge drivers.

Configuration could be done via config-as-code (e.g. via .gitattributes like Git supports) or in the application.

This may be tricky to do as under the hood we use merging to calculate the effective diff for PRs and you'd probably want to see the effective diff unmolested by the merge driver but then perform the merge using the merge driver.