Remove the usage of json inside genesis-related commands

[RiccardoM]

Context

Currently we're using a lot of json.Marshal and json.Unmarshal inside genesis-related methods (eg. DefaultGenesis, VerifyGenesis and InitGenesis). This is done due to cosmos/cosmos-sdk#8333.

Although it works, it makes it incredibly hard to manage things, as we need to always remember what to use (json vs cdc).

Implementation proposal

I think we should remove all the gogoproto.jsontag usage from our Proto file definitions, and revert to using only cdc instead. This would make it easier and simplify a lot of things in the future.