Reduce complexity of SerializeCosmosTx/DeserializeCosmosTx

[seantking]

Summary

At present, the serialization/deserialization of transactions is overly complex. We can reduce complexity by forcing the TrySendTx API to only take an array of SDK messages rather than accepting a single SDK message.

DeserializeTx should be renamed to DeserializeCosmosTx

This task is dependent on #449

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[gjermundgaraba]

This issue was mistakenly reopened during some integration work, so closing again now.