bootstub: Add OWNERS file

This CL is populating OWNERS file based on the candidates in the Chrome OS OWNERS status sheet.

BUG=b:132095048
TEST=None

Change-Id: Id3b7f91a10d7fa5c3dab40680d0bfcfb260bf274
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/bootstub/+/1682606