TurnCustomizer - an interface for modifying stun messages sent by TurnPort

This patch adds an interface that allows modification of stun messages
sent by TurnPort. A user can inject a TurnCustomizer on the RTCConfig
and the TurnCustomizer will be invoked by TurnPort before sending
message. This allows user to e.g add custom attributes as described
in rtf5389.

BUG=webrtc:8313

Change-Id: I6f4333e9f8ff7fd20f32677be19285f15e1180b6
Reviewed-on: https://webrtc-review.googlesource.com/7618
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20233}

api/peerconnectioninterface.h

@@ -83,6 +83,7 @@
 #include "api/rtpsenderinterface.h"
 #include "api/stats/rtcstatscollectorcallback.h"
 #include "api/statstypes.h"
+#include "api/turncustomizer.h"
 #include "api/umametrics.h"
 #include "call/callfactoryinterface.h"
 #include "logging/rtc_event_log/rtc_event_log_factory_interface.h"
@@ -474,6 +475,12 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
     // interval specified in milliseconds by the uniform distribution [a, b].
     rtc::Optional<rtc::IntervalRange> ice_regather_interval_range;
 
+    // Optional TurnCustomizer.
+    // With this class one can modify outgoing TURN messages.
+    // The object passed in must remain valid until PeerConnection::Close() is
+    // called.
+    webrtc::TurnCustomizer* turn_customizer = nullptr;
+
     //
     // Don't forget to update operator== if adding something.
     //