Add special handling for IBC Transfer to destination callback

[chipshort]

closes #2398

This adds preparsed information that will only be filled if the packet that arrived is from an IBC transfer. I decided to add this to the message struct instead for simplicity and better discoverability.

[kulikthebird]

LGTM

[chipshort]

I've added some more information to the type.
There are usecases for the sender, e.g. when implementing some kind of forwarding, you might want to refund the sender if something fails.
And it seemed a bit arbitrary to me to only handle transfers where the contract is the receiver, so I dropped that limitation and instead add the receiver address, so the contract can check it itself.

[kulikthebird]

Good to go!

[chipshort]

I rebased and added a changelog entry