Add rust_protobuf module.

This adds a new SourceProvider module type to handle protobuf code
generation. See the new test for an example of how to call this.

Bug: 143953733
Test: New soong tests pass.
Test: Replacing genrules in crosvm with rust_protobuf modules.
Change-Id: Ie3117129cde37b8736bc18ee09bf5cde27c01c34