[owners] Add OWNERs files to flatbuffers Bug: 77449 Change-Id: I331537922e3f01b4a444112160e330aa83a6e045 Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/flatbuffers/+/532265 Reviewed-by: Chris Fontas <cfontas@google.com>