v3 Onion Services: Put IPv6 and unrecognised link specifiers in onion service EXTEND cells

Prop224 says:

The hidden service SHOULD NOT reject any LSTYPE fields which it
doesn't recognize; instead, it should use them verbatim in its EXTEND
request to the rendezvous point.

https://gitweb.torproject.org/torspec.git/tree/proposals/224-rend-spec-ng.txt#n1689

We should either remove this from the spec, or we should:

• add a similar sentence for client descriptor lspecs
• put unrecognised lspecs in descriptors in client intro EXTEND requests
• put unrecognised lspecs in INTRODUCE cells in service rend EXTEND requests