WIP: Ticket34069: allow extend_info_t to contain take multiple orports

This branch is part of Sponsor 55: it allows an extend_info_t to contain multiple addresses, all of which are placed into the extend cell.

It does not currently put multiple addresses into extend cells, however, or behave in any way smarter than "use the first". That's another ticket.

This branch also adds convenience functions for working with extend_info_t and a couple of other things, and refactors some code around usage of extend_info_t.

Closes #34069 (closed)