This change add parsing editions specific descriptor parts to
internal/filedesc which makes it possible to load file descriptors of
protos using editions at runtime.
Change-Id: I7a0c23303acddd2cff115859d4cf82bf0102b14c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/563615
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
