Add min_bitrate_bps to RTCRtpEncodingParameters.

This is an ObjC followup to https://webrtc-review.googlesource.com/c/src/+/78741.

This CL only adds the field to the API, but does not wire it up.

Bug: webrtc:9341
Change-Id: Id6b1ac681324120bc90158029da7a80bf99aa512
Reviewed-on: https://webrtc-review.googlesource.com/81182
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23524}

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

@@ -25,6 +25,13 @@ RTC_EXPORT
  */
 @property(nonatomic, copy, nullable) NSNumber *maxBitrateBps;
 
+/** The minimum bitrate to use for the encoding, or nil if there is no
+ *  limit.
+ *
+ *  Not implemented.
+ */
+@property(nonatomic, copy, nullable) NSNumber *minBitrateBps;
+
 /** The SSRC being used by this encoding. */
 @property(nonatomic, readonly, nullable) NSNumber *ssrc;