save PT bridge lines to disk

save PT bridge lines to disk

Solves https://gitlab.torproject.org/tpo/core/tor/-/issues/29128.

pt_update_bridge_lines() was based on pt_get_extra_info_descriptor_string().
In particular, I copied the behaviour of not printing IPv6 addresses if IPv4 is
present.  I'll make both functions use IPv6 in another MR.

That function is called when the IP or key changes.  Sadly, I don't remember
how I decided from where exactly to call it (I wrote this ages ago), but it
seems fine?