Add virtual custodian wallet to Standard Programs

[vicsn]

This is in extension to: #143

We should add a virtual wallet example for custodians, this is often asked about: ProvableHQ/leo-examples#39

We can then add the following text:

Aleo smart contracts support secp256k1-based ECDSA signature verification. It is therefore possible for a custodian to deploy a smart contract representing a virtual wallet, authorized with ECDSA signatures. An ephemeral throwaway Aleo private key must be used to sign such a transaction, but is otherwise not directly used for authentication. Such a virtual smart contract wallet can only hold public funds.