Merge rust_protobuf and rust_grpcio module types.

To allow grpc protobufs to include non-grpc protos in a singular
library, we need to allow them to be defined as part of the same module.

This CL merges the two previously distinct module types into
rust_protobuf, and adds a new property for declaring which protos
contain grpc definitions.

Bug: 172952239
Test: rust_grpcio modules converted to rust_protobuf modules and build.
Test: Example rust_protobuf module with both proto types builds.
Change-Id: I0e627fd97bc6f74de89d309e3344694a1e76586d