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

rust/rust_test.go

@@ -62,6 +62,7 @@ func testConfig(bp string) android.Config {
 		"foo.c":      nil,
 		"src/bar.rs": nil,
 		"src/any.h":  nil,
+		"buf.proto":  nil,
 		"liby.so":    nil,
 		"libz.so":    nil,
 	}