Support psetv2 taproot fields

[louisinger]

This PR adds input taproot fields in psetv2.Input struct and add support to sign and finalize them

TL;DR

• pset taproot fields on Input, serialization and deserialization
• add Updater methods associated with taproot fields
• add Signer methods to sign a tapscript input & sign with taproot key-path
• add default finalizers in Finalize for tapscripts leaves and key-path.
• add 2 psetv2 taproot test (1 tapscript checksig, 1 key-path)

it closes #220

@altafan please review