CrossChainSwaps: Add a middleware to handle IBC metadata for the swap

[nicolaslara]

Design a middleware that allows us to extend IBC transfers with the extra metadata needed for crosschain swaps:

• Tokens to swap into
• Callback?

On receiving the metadata, the middleware would then call a function to make the swap and transfer the seapped tokens back to the caller (or alternatively another address or contract if a callback is specified)

If possible it should be designed in generic way so that it can be upstreamed to ibc-go.

[p0mvn]

@nicolaslara when you get the time, do you mind linking all relevant PRs one could follow about this feature?

[nicolaslara]

@p0mvn I don't have a PR ready yet, but I'm working on nicolas/crosschain-swaps

But I've extracted a few of those changes into these IBC-go PRs:

cosmos/ibc-go#2296
cosmos/ibc-go#2306
cosmos/ibc-go#2305

Depending on what changes in ibc-go we may need more or less on our end

[ValarDragon]

Done!