[Feature request] Add Proxy Protocol support

[ArnGa]

Hello, When Unbound is behind a DNS proxy, the client's IP address cannot be used by Unbound (rpz log or ACL for example). To correct this problem, it could be interesting to develop the support of the "Proxy Protocol" in Unbound in order to really take into account the positioning of Unbound in backend. The "Proxy Protocol" was developed by HAProxy (https://www.haproxy.org/download/2.2/doc/proxy-protocol.txt) but it is agnostic and is not specific to HTTP. The DNS proxy DNSDist implements this protocol in its latest version (https://dnsdist.org/advanced/proxyprotocol.html) and it would be a big plus if it was possible to make Unbound compatible.

Regards

[alexrsagen]

This could potentially also be useful for unbound DoH behind a reverse proxy.

[gthess]

This is closed by #760; PROXYv2 is part of Unbound 1.17.0.

[gthess]

For DoH that could be another feature where HTTP headers are used instead of PROXYv2 to pass along client information.