add CODEOWNERS file for each repo

[michelleN]

Currently we have maintainers files which were easier to manage under the fermyon github org where access control for spin repos for non Fermyon employees was a little tricky. We can probably replace those with CODEOWNERS files now. CODEOWNERS files will be easier to manage and keep up to date. They also allow for us to automatically request reviews from the right people for the right parts of the codebase.