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