proto: re-export prost_types as well_known_types

My main interest is being able to get to `prost_types::Timestamp`
without having to include `prost_types` in addition to
`tendermint-proto`, however I think there might be more cases where it's
helpful to have access to the well-known proto types in the future.

proto/src/lib.rs

@@ -86,6 +86,7 @@ mod tendermint {
 }
 
 pub use tendermint::*;
+pub use prost_types as well_known_types;
 
 mod domaintype;
 pub use domaintype::DomainType;