Add governance proposal callback hooks

[ethanfrey]

Support the new messages added in confio/tgrade-contracts#2

[alpe]

I do understood this task that we want to handle any ExecuteGovProposal returned from a contract.

While implementing this though, the RawProtoProposal type opens the door for another interpretation or feature where a contract registers for a callback to receive this type from the gov handler.

[ethanfrey]

While implementing this though, the RawProtoProposal type opens the door for another interpretation or feature where a contract registers for a callback to receive this type from the gov handler.

I don't understand the how this works? Is this a feature or a bug? What exactly could we do? I assume we can only call registered gov/proposal handlers and this RawProtoProposal only works to call a chain-specific gov proposal handler, which was not in our list. That said, this is only designed for tgrade, so maybe best to just remove that one and if we forgot something, add it as an explicit enum to the tgrade bindings