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}

examples/objc/AppRTCMobile/ARDAppClient.m

@@ -533,8 +533,14 @@ static int const kKbpsMultiplier = 1000;
   // Create peer connection.
   RTCMediaConstraints *constraints = [self defaultPeerConnectionConstraints];
   RTCConfiguration *config = [[RTCConfiguration alloc] init];
+  RTCCertificate *pcert = [RTCCertificate generateCertificateWithParams:@{
+    @"expires" : @100000,
+    @"name" : @"RSASSA-PKCS1-v1_5"
+  }];
   config.iceServers = _iceServers;
   config.sdpSemantics = RTCSdpSemanticsUnifiedPlan;
+  config.certificate = pcert;
+
   _peerConnection = [_factory peerConnectionWithConfiguration:config
                                                   constraints:constraints
                                                      delegate:self];