Better Native ICA support

Use case: _I want to write a contract that stakes an asset on a non-CosmWasm chain._

Really love implementing ICA in pure CosmWasm as with Confio's [cw-ibc-demo](https://github.com/confio/cw-ibc-demo). Unfortunately this will not work with non-CosmWasm chains like the Cosmos Hub.

Reading through the latest [ICA documentation](http://tutorials.cosmos.network/academy/4-ibc/ica.html), it seems we may need to call [RegisterInterchainAccount](https://github.com/cosmos/ibc-go/blob/main/modules/apps/27-interchain-accounts/host/keeper/account.go) and then can call `SendTx`? Or perhaps just `SendTx` is enough and it's smart enough to register the account?

Would be nice if we could extend `cosmwasm_std` [IbcMsg](https://docs.rs/cosmwasm-std/1.0.0/cosmwasm_std/enum.IbcMsg.html) to support native ICA messages. I'm hacking a bit today on getting them working using `CosmosMsg::Stargate`, but not sure how successful that will be and that it could be a much better dev experience to just support it outright.

In `cosmwasm_std` it would be ultimately nice to have something like:
```rust
#[non_exhaustive]
pub enum IbcMsg {
// ...
IcaTx {
channel_id: String,
msgs: Vec>,
timeout: IbcTimeout,
},
// Guess we need this to get the channel ID?
RegisterIca {
connnection_id: String,
port_id: String,
address: String,
}
}
```

Realize this issue also touches the `cosmwasm` repo, but figured it would be best to start the conversation here.