Add CODEOWNERS file

[dbluhm]

It seems like it would be wise to add a CODEOWNERS file to the repo. From my understanding of how that works, it would automatically tag code owners for PRs touching the code they have ownership over. Especially given the multi-project nature of this repo, this should help get the right people's attention on the PR. I could review changes to multitenant_provider but it'd probably be more beneficial for someone from the BC Gov team to review it, for instance.