Change ics20 packet data to use standard json library for marshaling

[colin-axner]

Summary

Change ICS 20 packet data FungibileTokenPacketData function GetBytes to use "encoding/json" library, rather than the sdk proto codec function MustMarshalJSON

Problem Definition

The ibc spec states that the ics20 packet data should be JSON marshaled, it does not state it should be proto3 json marshaled. To avoid accidental non-conformance when adding additional fields to the existing packet data we should switch from the proto codec MustMarshalJSON (which uses proto3 json) to the standard library json.Marshal.

The change would not be breaking because proto3 json maps strings to json strings (all fungible token packet data fields are strings)

In addition, this would remove the dependency on sdk.MustSortJSON which is being deprecated. The sorting of the json could be removed regardless since sorting of keys does not affect the ability of the counterparty to unmarshal the packet data

This also removes the maintenance burden of the modified MustMarshalJSON function. The standard library handles not emitting empty defaults via the omitempty json tag which is included by default on our .pb.go (we should ensure this doesn't change)

Proposal

Change FungibleTokenPacketData.GetBytes() to use json.Marshal(). Add documentation to the function to explain the usage of omitempty

In addition, when cosmos/ibc#987 is addressed, our marshal should link to the encoding spec

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[colin-axner]

We should probably also decode the packet using the standard json library