Expose different consts, types and impls for public access

[Farhad-Shabani]

Background

Part of #965

To enable the ibc restructure and also maximize users' access to our implementation, we'll be making the following types and implementations public:

• validate and execute handler functions for all the IBC message types.
• TYPE_URL constants.
• Any private fields within the domain message types.
• Any private fields within the Tendermint ClientState and ConsensusState structs.