[Splicing] Support shared signature in InteractiveTransaction

[optout21]

Splicing needs support for shared signatures during interactive transaction negotiations -- when there is an input that has to be signed by both peers (the previous multisig funding tx becomes an input to the new funding tx).
This is a relatively minor extension, in preparation for splicing ( #1621 ).

Prerequisites: #3137

Changes can be seen in #3274 , changes to file interactivetxs.rs, however, not very clear as there are other changes as well. Search for shared, splic.