Introduce access-control scheme for interacting with a smart contract.

[rkeene]

We want to support the use case where a user with no balance may call a smartcontract that is so defined, alternatively put that a user's balance is not considered or affected with a call to a smartcontract is so defined. Instead, a contract-specified or user-specified wallet is used to fund the transaction.

Similar to: https://medium.com/tabookey/1-800-ethereum-gas-stations-network-for-toll-free-transactions-4bbfc03a0a56

Additionally, this use case requires that either only certain public keys or all public keys be permitted to engage with this functionality of the specific smart contract. Some mechanism must exist to for the "funding-wallet" to authorize its use by the smartcontract as well.

Indeed the mechanism may be a new type of message where wallets permit contracts to pull funds from their account for specified other accounts.

This is an investigation into what it would take, and what the resulting feature would look like, and if it should be pursued.