Add a UnmarshalPacketData optional interface function to support ADR 8 callbacks

[colin-axner]

Summary

In order for ADR 8 to work generically with the given middleware construction, it needs to ask the underlying app to decode the packet data bytes into an interface function.

Problem Definition

Middleware has no way of generically decoding packet data bytes without making assertions on the underlying app

Proposal

Add an optional interface which IBC apps can implement. It should take in bytes and return an interface. For ICS 20 the FungibleTokenPacketData should be returned. For ICS 27 the InterchainAccountsPacketData should be returned. ADR 8 can cast these interfaces to the CallbacksPacketData if supported.

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[colin-axner]

closed by #3353