Add @cosmjs/cli helpers for each contract

[ethanfrey]

Add some helper code that will allow simple query/execute of a given contract on the client side.

The basic idea is similar to web3x-codegen, but hand-coded for now. Wrapping a CosmWasmSigningClient and providing methods for each HandleMsg and QueryMsg, possibly init and upload methods.

In a future version, when we like the API, we can see how to auto-generate this from the JSON Schema

Related to cosmos/cosmjs#385

[ethanfrey]

Done for cw20-base and cw1-subkeys. We can look at adding more once 0.23 is released