Implement RtpCodecParameters::parameters

This will return all the fmtp parameters for the codecs, except for
DTMF codes that don't fit the key=value pattern.

Bug: webrtc:7112
Change-Id: I06a203ff64df2c3bc9bc2082cd0f374718b23510
Reviewed-on: https://webrtc-review.googlesource.com/71801
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23250}

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

@@ -63,6 +63,9 @@ RTC_EXPORT
  **/
 @property(nonatomic, readonly, nullable) NSNumber *numChannels;
 
+/** The "format specific parameters" field from the "a=fmtp" line in the SDP */
+@property(nonatomic, readonly, nonnull) NSDictionary *parameters;
+
 - (instancetype)init NS_DESIGNATED_INITIALIZER;
 
 @end