Refactor 'UnmarshalPacketData' in interchain accounts

[srdtrk]

Summary

UnmarshalPacketData explicitly deserializes packet data from bytes to a concrete type. This logic should be moved to types and other parts of the code can just call this logic in icahost and icacontroller (including UnmarshalPacketData).

Problem Definition

UnmarshalPacketData allows middlewares higher in the stack to request the module to deserialize the packet data. This logic can be used to refactor other parts of the code if it is moved to types. We also have the long term goal of making ModuleCdc private and just expose the serialization and deserialization functions to the public API. We would eventually do this for all apps but we first implement this in interchain accounts.

Proposal

1. Move UnmarshalPacketData's logic to types.
2. Refactor other areas of the interchain accounts code to use this function when possible.

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