Modify ics20 proto codec to not emit unpopulated fields

[colin-axner]

Summary

In order for ics20 to add a memo field in a backwards compatible fashion we need to not marshal unpopulated fields (memo) on the sending chain. This was the expected behaviour, but upon further investigation by @damiannolan, we have learned the SDK sets EmitDefaults and subsequently EmitUnpopulated to true

We should copy/simplify the ProtoMarshalJSON function and set EmitDefaults to false. This should only be used by ics20 packet data GetBytes function

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[damiannolan]

I'm pretty happy with this as a solution for now tbh. It's actually probably as clean as it gets for a short term patch.
Great work @colin-axner @AdityaSripal :)