Added CODEOWNERS

[wknapik]

Part of #12

[fmarier]

Instead of using CODEOWNERS, could we just use the regular review restrictions like we do on brave-core?

i.e. require 1 approving review, and then add as collaborators only the people who should

The main benefits I see is that we don't have another file to maintain, and if I go on holiday, you can still get your PRs reviewed & merged.

[wknapik]

We require code owner review in brave-core and I'd prefer to have that here as well.

Access to the repo can change over time and a random review is not what we're looking for. This repo will be very sensitive, so I'd prefer to take every precaution.

In the long run, I'd also add the security team and (part of) the devops team as code owners. Can't do that now, while the repo is in brave-experiments.