Base64 of the JSON {"action":"SPONSOR_GAS","date":<unix seconds>} signed
by walletAddress. Proves who is asking, not what for. Reusable for a day
either side of date, so sign once and cache it.
walletAddress's signature over bytes.
OptionaltxSerialized transaction kind to sponsor, when the request includes a transaction.
Wallet requesting sponsorship.
Inputs for requesting a gas-pool-sponsored transaction.