Better support/docs for calling contracts from native code

[ethanfrey]

This came up in some discord discussions. It is totally possible now if you get the x/wasm.Keeper or the Handler. We do enable a two way street (contract -> native and native -> contract). However, the second path is not well documented. Let's show a demo somewhere and add some API helpers as needed

[alpe]

This is what we have implemented in tgrade.