Create a doc to list ORAS subproject owners

[FeynmanZhou]

CNCF uses the maintainer list as a reference to grant permissions to the project maintainers in some CNCF events or programs. However, there are only ORAS Org Owners listed on the CNCF maintainer list, which blocks the subproject owners to collaborate and engage with those CNCF programs, see this issue as an example.

I propose creating a document subproject-owners.md to list all of the ORAS subproject owners in a central place, i.e. in this repository, then sync it to the CNCF maintainer list.