Add public API for Diffie-Hellman Tuples transaction signing

[arobsn]

With the new privacy protocols like Stealth Addresses seems necessary to add to have a public API for DHT transaction signing so that wallets and dApps can work with such protocols.

I'm not sure about the API specification, but accepting params as UInt8Array would be great, as it can be compatible with keys generated from outside the ErgoLib.

Additionally, here is how node's sign endpoint requires DHT params:

dht: [
  {
    secret: string;
    g: string;
    h: string;
    u: string;
    v: string;
  }
]