This PR updates the protos to match the recent changes in
https://github.com/apple/container-builder-shim/pull/15.
This PR additionally adds the builder shim version as a variable in
Package.swift. This allows us to be consistent with the builder tag used
for the builder shim image and when building protobuf files.
Signed-off-by: Kathryn Baldauf <k_baldauf@apple.com>