ibc: add memo field to ICS20 FungibleTokenPacketData

[avahowell]

Is your feature request related to a problem? Please describe.
Since the implementation of cosmos/ibc#584, the standard packet data for ICS20 transfers adds a new field, memo: string, in order to support more complex uses of interchain transfers. Our ICS20 implementation was written before this change was widely adopted by the ecosystem (as it is now), and as such doesn't contain this memo field in FungibleTokenPacketData.

Describe the solution you'd like
We should upgrade our IBC protos to the current interchain standard that includes the memo field. I don't think we need to populate it in our state machine, just updating the protos (and any associated domain types) should be sufficient. This change should be implemented in ibc-types.