Add option to use protobuf3

Users of the rust_protobuf module can use the 'use_protobuf3'
option to select version 3 of the protobuf crate instead of the
current default of version 2. This unblocks teams that would like
to use protobuf 3 immediately and provides a mechanism for us to
incrementally migrate users of protobuf 2 to protobuf 3.

Test: Adds tests that are executed during every build.
Bug: 270895633
Change-Id: Ib6a18b77cfa7dc3cc604cea05269004b3712bbb6