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}

sdk/android/api/org/webrtc/PeerConnection.java

@@ -304,6 +304,9 @@ public class PeerConnection {
     public int maxIPv6Networks;
     public IntervalRange iceRegatherIntervalRange;
 
+    // This is an optional wrapper for the C++ webrtc::TurnCustomizer.
+    public TurnCustomizer turnCustomizer;
+
     // TODO(deadbeef): Instead of duplicating the defaults here, we should do
     // something to pick up the defaults from C++. The Objective-C equivalent
     // of RTCConfiguration does that.