net: support overriding the proxy selection in ConnectNode()

Normally `ConnectNode()` would choose whether to use a proxy and which
one. Make it possible to override this from the callers and same for
`OpenNetworkConnection()` - pass down the proxy to `ConnectNode()`.

Document both functions.

This is useful if we want to open connections to IPv4 or IPv6 peers
through the Tor SOCKS5 proxy.

Also have `OpenNetworkConnection()` return whether the connection
succeeded or not. This can be used when the caller needs to keep track
of how many (successful) connections were opened.

src/test/fuzz/connman.cpp

@@ -177,7 +177,7 @@ FUZZ_TARGET(connman, .init = initialize_connman)
                     /*addrConnect=*/random_address,
                     /*fCountFailure=*/fuzzed_data_provider.ConsumeBool(),
                     /*grant_outbound=*/{},
-                    /*strDest=*/fuzzed_data_provider.ConsumeBool() ? nullptr : random_string.c_str(),
+                    /*pszDest=*/fuzzed_data_provider.ConsumeBool() ? nullptr : random_string.c_str(),
                     /*conn_type=*/conn_type,
                     /*use_v2transport=*/fuzzed_data_provider.ConsumeBool());
             },