[ISL][Feature Request] Specify github reviewers

Currently whenever creating a PR with sapling, I still need to open the PR in github to finish off some things. It would be much more ideal if I could be done after clicking "Submit". One such thing is specifying reviewers for the PR (which seems to be supported internally).

It seems like there's already support for something like this by adding to OSSDefaultFieldSchema and using the type: 'field'. Autocomplete could be easily implemented via github graphql API. My main concern is how to actually pass the information from ISL webview to lower level APIs (not very familiar with that), and vice-versa for PRs with reviewers. It seems like there's an assumption that commit description has all this metadata stored as text which seems weird for github PRs. If there's any quick pointers here I'd be happy to tackle this, but might be a bit complex.