function getTapKeySigFromWitness(finalScriptWitness) {
  if (!finalScriptWitness) return;
  const witness = finalScriptWitness.slice(2);
  // todo: add schnorr signature validation
  if (witness.length === 64 || witness.length === 65) return witness;
}
Gets the taproot signature from the witness.
@param finalScriptWitness The final script witness.
@returns The taproot signature, or undefined if not found.