Add config to prune low-priority TURN ports for creating connections
When the flag prune_turn_ports is set, When a high-priority turn port becomes available, it will prune low-priority ones. The pruned port will not be used for creating connections locally and its candidates will not be sent over to the remove side (unless they have been sent before being pruned).

This effectively reduces the number of TURN candidates and connections created by TURN ports.

BUG=
R=deadbeef@webrtc.org, pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/2093623004 .

Cr-Commit-Position: refs/heads/master@{#13335}
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index c72e2ec..382a4b4 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -300,6 +300,7 @@
     rtc::Optional<bool> combined_audio_video_bwe;
     rtc::Optional<bool> enable_dtls_srtp;
     int ice_candidate_pool_size = 0;
+    bool prune_turn_ports = false;
   };
 
   struct RTCOfferAnswerOptions {