Add certificate generate/set functionality to bring iOS closer to JS API

The JS API supports two operations which have never been implemented in
the iOS counterpart:
 - generate a new certificate
 - use this certificate when creating a new PeerConnection

Both functions are illustrated in the generateCertificate example code:
 - https://developer.mozilla.org/en-US/docs/Web/API/RTCPeerConnection/generateCertificate

Currently, on iOS, a new certificate is automatically generated for
every PeerConnection with no programmatic way to set a specific
certificate.

Work sponsored by |pipe|

Bug: webrtc:9498
Change-Id: Ic1936c3de8b8bd18aef67c784727b72f90e7157c
Reviewed-on: https://webrtc-review.googlesource.com/87303
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24276}

sdk/objc/Framework/Headers/WebRTC/RTCConfiguration.h

@@ -10,6 +10,7 @@
 
 #import <Foundation/Foundation.h>
 
+#import <WebRTC/RTCCertificate.h>
 #import <WebRTC/RTCMacros.h>
 
 @class RTCIceServer;
@@ -67,13 +68,15 @@ typedef NS_ENUM(NSInteger, RTCSdpSemantics) {
 };
 
 NS_ASSUME_NONNULL_BEGIN
-
 RTC_EXPORT
 @interface RTCConfiguration : NSObject
 
 /** An array of Ice Servers available to be used by ICE. */
 @property(nonatomic, copy) NSArray<RTCIceServer *> *iceServers;
 
+/** An RTCCertificate for 're' use. */
+@property(nonatomic, nullable) RTCCertificate *certificate;
+
 /** Which candidates the ICE agent is allowed to use. The W3C calls it
  * |iceTransportPolicy|, while in C++ it is called |type|. */
 @property(nonatomic, assign) RTCIceTransportPolicy iceTransportPolicy;