Allow InteractiveTxConstructor use in splicing

InteractiveTxConstructor was only used in PendingV2Channel methods, but
for splicing those methods are needed for FundedChannel, too. Refactor
the code such that each type has a method for accessing its
InteractiveTxConstructor such that it can be called in either use,
refactoring code out of PendingV2Channel as needed.

Co-authored-by: Wilmer Paulino <wilmer@wilmerpaulino.com>
Co-authored-by: Jeffrey Czyz <jkczyz@gmail.com>

Author: wpaulino