Query code by checksum

It would be cool to have a query like this:

```
// Code gets the binary code and metadata for a singe wasm code
rpc Code(QueryCodeRequest) returns (QueryCodeResponse) {
option (google.api.http).get = "/cosmwasm/wasm/v1/code/{code_id}";
}
```

but with the checksum as an argument.

The use case for this is as follows: I want to instantiate a contract and upload it only when it does not yet exist. If the checksum exists already, I can use the existing one.