Add signPsetV2 and blindPsetV2 to identities

[altafan]

This adds 2 new methods to identities signPsetV2 and blindPsetV2 to manage transactions with the new psetv2 format.

The base Identity type has a new function blindPsetV2WithSource that basically takes care of creating the xkp blinding generator either from a list of unblinded inputs, a master blinding key or a list of blinding private keys by giving priority in this exact order in case more than one source is defined.

NOTE: The method blindPsetV2 accepts an optional list of unblinded inputs that the user can pass in case he already unblinded them all. Otherwise the master key/list of blinding keys of the identity will be used to unblind the tx inputs if needed.

Closes #125.

Please @tiero @louisinger review.

[altafan]

@louisinger I merged new commits of master to this PR but now CI always fails with this min-relay-fee-not-met error in a test that I didn't touch at all. Can you check if I've messed up with something with my merge?

[altafan]

@tiero @louisinger I fixed the issues with tx estimation by making a little refactor of the core logic.

Note that at the moment the estimation assumes all inputs are of type p2wpkh | p2sh(p2wpkh), therefore I left a TODO with a brief description of what to do to be more generic and make an accurate estimation with all kind of (non-taproot) inputs.

[tiero]

Maybe here we should decode the script type? if is segwit v1 we should branch out, and at least if is a key-path spend try to make the schnorr signature necessary?

[tiero]

If not in this PR, at least let's throw an error that is not implemented in the case is a taproot output and/or input.tapLeafScript is present, so user knows can only signs segwit v0

[tiero]

@altafan @louisinger