In this commit, we fix an inadvertent mutation bug that would at times
cause the private key passed into the tweak function to actually be
*modified* in place.

We fix this by accepting the value instead of a pointer. The actual
private key struct itself contains no pointer fields, so this is
effectively a deep copy via dereference.

We also add a new unit test that fails w/o this change, to show that the
private key was indeed being modified.

This commit adds a GetTxFee method to the psbt.Packet structure. The method returns the PSBT's fee.

btcutil/psbt: add method Packet.GetTxFee

txscript: modify TweakTaprootPrivKey to operate on private key copy

This is a fix similar to #1905.
We'll always make a copy of the key in the local scope before passing it
around elsewhere. Depending on the parity of the public key, the private
key itself might need to be negated.

A similar test is added here that fails without the patch to the
signature.go file.

btcec/schnorr: use private key copy for BIP-340 signatures