improve descriptor arguments published

This is the extra-info line I see for the bridge documented in the README:

transport webtunnel 127.0.0.1:11000 domainname=n97jlbvjwy3iotetc0axrtu3mm7gq1gajqv7-80-78-24-44.sslip.io,path=939e42a2-8f3f-4645-9d7b-8b9be28b157e,port=443,publicVersion=0.0.1

Currently webtunnel is publishing the localhost IP and port where is listening for connections from the http proxy, we might want to publish a different IP address and port there as they will be used to build the bridgeline. The README uses 192.0.3.2:1, but AFAIK we can't use the same IP-port combination for several bridges as tor will consider is the same bridge and ignore the others. We could generate a random IP address from a private range, but maybe is better to put there the public IP address of the domain we are using as front. Any ideas?

I see in the extra-info line domainname and path, where AFAIK the client expects url. I think this is not coming from the code itself, but from ServerTransportOptions in torrc. Isn't it? Maybe we can improve the README to document how that is configured to include the url option (and fix our public tunnel).

I have some doubts about publicVersion, I think is weird to keep that in the bridgeline. rdsys could strip it out, but is pretty hacky. I would love to see tpo/core/tor#11101 solving that problem instead and