Add Onionbalance Support

[cretz]

At least need an example showing it, if not helpers to make it easy.

[cretz]

Onionbalance for v3 relies on a recent PR at https://github.com/torproject/tor/pull/1647 (discussion at https://gitlab.torproject.org/tpo/core/tor/-/issues/32709) which only applies to torrc-based onion services, not ephemeral ones via ADD_ONION. Mailing list discussion started at https://lists.torproject.org/pipermail/tor-dev/2021-June/014570.html.

I may also look into impl'ing a poc of this PDF which discusses multiple instances using the same hostname and priv key.