Backup protos

Swift-Protobuf

We use Swift-Protobuf to generate Swift code for working with Backup.proto, much like we do for all our other proto definitions.

However, unlike (most of) the other protos, we do not use the "wrapper" code generated by $REPO_ROOT/Scripts/protos/ProtoWrappers.py to wrap the Swift-Protobuf-generated code for Backups because ProtoWrappers.py is incompatible with some of the syntactic structures we use in Backup.proto.