Add set_tunnel_hostname_addr_map() and deprecate set_hostname_addr_map() (#2647)

Through a CONNECT proxy it is the proxy that connects to the origin, and the CONNECT target was always the original hostname, so there was no way to send the tunnel to another address. set_tunnel_hostname_addr_map() sets that target on SSLClient, Client and WebSocketClient. The CONNECT request's Host follows it, including the Digest retry, while the Host header, the SNI and the certificate check inside the tunnel keep the original hostname. The map is also copied to a redirect client.

set_hostname_addr_map() is deprecated in favor of set_direct_hostname_addr_map(), which behaves the same and names the connection it applies to. The two maps are independent and correspond to curl's --resolve and --connect-to; neither applies to the proxy's own address.

docs-src/pages/en/tour/09-whats-next.md

@@ -214,7 +214,7 @@ svr.listen("/tmp/httplib.sock", 80);  // The port is unused, but must not be 0
 // Client
 httplib::Client cli("http://localhost");
 cli.set_address_family(AF_UNIX);
-cli.set_hostname_addr_map({{"localhost", "/tmp/httplib.sock"}});
+cli.set_direct_hostname_addr_map({{"localhost", "/tmp/httplib.sock"}});
 
 auto res = cli.Get("/");
 ```