mediafoundation: add support dynamic bitrate control

I use h264 encoder in UWP app. I can set 'bitrate' value only when pipeline is stopped. I want to control bitrate of video stream dynamically during playing state (encoder CBR mode). I tried to set 'bitrate' and 'max-bitrate' in 'rc-mode' 0 and 1, but it is not changed. I use gstreamer 1.18.4 UWP. As far as I understand this is not supported by gstreamer now. This is feature request issue. Thanks!