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}
diff --git a/AUTHORS b/AUTHORS
index 69b5e2f..41833d0 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -36,6 +36,7 @@
 Martin Storsjo <martin@martin.st>
 Matthias Liebig <matthias.gcode@gmail.com>
 Maxim Potapov <vopatop.skam@gmail.com>
+Michael Iedema <michael@kapsulate.com>
 Mike Gilbert <floppymaster@gmail.com>
 Mo Zanaty <mzanaty@cisco.com>
 Pali Rohar