Function getWitnessSize

Calculates the value of a multisig witness given m-of-n values

According to Bitcoin Optech calculator (https://bitcoinops.org/en/tools/calc-size/):

• P2WSH 2-of-3 witness: 63.5 vbytes (254 bytes / 4)
• Format: (1) size(0) (empty) + (73) size(72) ecdsa_signature + (73) size(72) ecdsa_signature + (106) size(105) redeem_script

Witness structure:

• Witness item count (varint)
• Size
• m signatures, each with size prefix
• Redeem script with size prefix