Add monorepo support

[jonbodner]

Right now, a single rule is selected to determine whether or not checks-out see sufficient approvals to accept a PR. When working in a monorepo, this isn't sufficient; different paths in the same project will have different approval rules, and you don't know ahead of time which combination of them need to be active for a given PR. Adding the ability to specify that multiple approval rules can be active at once, determined per-PR would enable a flexible monorepo approval system.

[shusak]

If #60 was done - could this be resolved by that? at least from a check on who?

[jonbodner]

My quick scan of how CODEOWNERS works is that is per-branch. ~The way I am thinking about it, monorepo support needs to be per-path. But I can be convinced that I am wrong.~ I was wrong; we can use this to specify owners per-path, per-branch.

I have a different potential early implementation of monorepo support, but I need to find out from the Open Source Office how to get the code here. But maybe it should be abandoned to meet this instead.