Add rust_grpcio module type.

Adds a new SourceProvider type to generate grpcio code from protos.
Since it's so similar to protobuf, it's basically just a different type
of rust_protobuf.

Bug: 171504899
Test: Example module compiles.
Change-Id: I9882f3ac4d4aeaae0191f2b557e9612b5c7a9ac9
diff --git a/rust/testing.go b/rust/testing.go
index 42b0da1..4a1894c 100644
--- a/rust/testing.go
+++ b/rust/testing.go
@@ -132,6 +132,8 @@
 	ctx.RegisterModuleType("rust_ffi_host", RustFFIHostFactory)
 	ctx.RegisterModuleType("rust_ffi_host_shared", RustFFISharedHostFactory)
 	ctx.RegisterModuleType("rust_ffi_host_static", RustFFIStaticHostFactory)
+	ctx.RegisterModuleType("rust_grpcio", RustGrpcioFactory)
+	ctx.RegisterModuleType("rust_grpcio_host", RustGrpcioHostFactory)
 	ctx.RegisterModuleType("rust_proc_macro", ProcMacroFactory)
 	ctx.RegisterModuleType("rust_protobuf", RustProtobufFactory)
 	ctx.RegisterModuleType("rust_protobuf_host", RustProtobufHostFactory)