Allow SignatureTemplate signing of non-transaction messages

[mr-zwets]

There's a TODO for this in the code in SignatureTemplate.ts

  // TODO: Allow signing of non-transaction messages (i.e. don't add the hashtype)
  generateSignature(payload: Uint8Array, bchForkId?: boolean): Uint8Array {
    const signature = this.signatureAlgorithm === SignatureAlgorithm.SCHNORR
      ? secp256k1.signMessageHashSchnorr(this.privateKey, payload) as Uint8Array
      : secp256k1.signMessageHashDER(this.privateKey, payload) as Uint8Array;

    return Uint8Array.from([...signature, this.getHashType(bchForkId)]);
  }

[mr-zwets]

Currently, without this CashScript option, devs need to use the libauth primitives for this, which requires manual conversions of the privateKey type into Uint8Array

import { secp256k1 } from '@bitauth/libauth';
import { sha256 } from '@cashscript/utils';

signMessage(message: Uint8Array, privateKey: Uint8Array): Uint8Array {
    const signature = secp256k1.signMessageHashSchnorr(privateKey, sha256(message));
    if (typeof signature === 'string') throw new Error();
    return signature;
  }