Fakedns fix xUDP destination override (#1011)

* Fix UDP destination override

* Fix code style

* Fix fakedns object init

Do type convertion at runtime in case if user don't use fakedns in config.
Since dispatcher now depend on fakedns object, move the injection order of
fakedns to top (As a temporary solution)

* Amend logic for handing fakedns client

A map is used by server side when client turn on fakedns
Client will send domain address in the buffer.UDP.Address, server record all possible target IP addrs.
When target replies, server will restore the domain and send back to client.

Co-authored-by: hmol233 <82594500+hmol233@users.noreply.github.com>

transport/pipe/pipe.go

@@ -3,6 +3,7 @@ package pipe
 import (
 	"context"
 
+	"github.com/xtls/xray-core/common/buf"
 	"github.com/xtls/xray-core/common/signal"
 	"github.com/xtls/xray-core/common/signal/done"
 	"github.com/xtls/xray-core/features/policy"
@@ -25,6 +26,12 @@ func WithSizeLimit(limit int32) Option {
 	}
 }
 
+func OnTransmission(hook func(mb buf.MultiBuffer) buf.MultiBuffer) Option {
+	return func(option *pipeOption) {
+		option.onTransmission = hook
+	}
+}
+
 // DiscardOverflow returns an Option for Pipe to discard writes if full.
 func DiscardOverflow() Option {
 	return func(opt *pipeOption) {