JS client to send transactions to the sequencer

[szynwelski]

The client should allow the creation of a Cosmos transaction containing a given signed Arweave transaction. The signature of the Cosmos transaction should be equal to the signature of the Arweave transaction (see #8).

After creating a transaction, the client should send it to the sequencer network.

Client should be as light as possible because it will eventually be included in Warp Contracts SDK.

To be decided:

• should we use CosmJS or write code from scratch
• whether to send a transaction encoded with protobuf via RPC or in the form of JSON via REST API
• could the client only send an Arweave transaction and a Cosmos node would wrap it in a Cosmos transaction

[szynwelski]

Decision:

• we add an endpoint to the REST API that accepts Arweave transactions in the form of JSON
• on the server side, we wrap it with a Cosmos transaction
• on the client side we do not use CosmJS and we do not encode transactions with protobuf

[szynwelski]

Centralized and decentralized sequencer client: warp-contracts/warp#472