Handle both `stargate` and `any` CosmosMsg variant

[chipshort]

Companion to CosmWasm/cosmwasm#1926

Unfortunately I did not find a succinct way to do this in Go. The other option would be to keep both variants in the struct, make them private and add a Stargate / Any function that differentiates them. But that would also need changes in wasmd.

[webmaster128]

That's the way! We take the burden on us to make the lifes of our users easier (wasmd in this case).