Refactor common code out of setup_introduce1_data and intro point functions

During #23577 (moved), we discovered that there's a lot of code in setup_introduce1_data() that's duplicated in service_intro_point_new() and hs_desc_lspec_to_trunnel().

In #23576 (moved), we removed the duplication across setup_introduce1_data() and service_intro_point_new(), creating node_get_link_specifier_smartlist().

So we should clean that up at some point, but it's complicated, because the intro point functions use hs_desc_link_specifier_t. (See #22781 (moved))

Edit: update after #23576 (moved).