Simplify json encoding of Execute/InstantiateMsg

Summary

MsgInstantiateContract and MsgExecuteContract, both take a message for the wasm contract as []byte. Let's change that to json.RawMessage.

Problem Definition

Msg is json inside all cosmwasm contracts so far, and additional tooling, like cw-storage makes that implicit assumption. Without limiting in any way the message format, we can just assert this is json.RawMessage. This makes embedding and creating such a structure much easier.

Assume the message is {"approve":{}}. As is, when we create MsgExecuteContract, it will have a base64 encoded field, something like:

{
  "contract": "cosmos1qlc8f2fmw",
  "msg": "eyJhcHByb3ZlIjp7fX0=",
}

this would make it much simpler, without modifying the binary encoding at all.

{
  "contract": "cosmos1qlc8f2fmw",
  msg: {"approve":{}},
}

Which one is easier to understand, create, and debug?

If a chain does want to allow contracts to use another codec, they can simply modify this portion and it will enable eg. protobuf or cap't proto, just with a bit of dev ux impact.

Proposal

• Update the field and ensure this works properly.
• Manually check both binary and json amino encoding before and after.
• Test the cli tools, and in particular viewing such a tx as json (both before and after) to ensure this is a nice enhancement to the UI.

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned