Stargate gaiad should have the option to emit signed Amino encoded TX from the tx sign and tx multisign commands

[zmanian]

In the Stargate version of the cosmos SDK, we accept signed AminoJSON encoded transactions via :1317/txs endpoint but there isn't an included tool to demonstrate what this JSON should look like.

I've prototyped a simple mechanism that uses a flag on tx sign and tx multisign to output AminoJSON encoded JSON instead of JSONEncoder() txs.

It's a pretty natural place for this and should help developers with porting.

Just asking for comments before making a PR.

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[alexanderbez]

/cc @aaronc