PSA: Changed argument type for PeerConnectionInterface::SetBitrate

[Niels Moller]

The struct PeerConnectionInterface::BitrateParameters is being
replaced by BitrateSettings, declared in
api/transport/bitrate_settings.h. The reason for the change is that we
had three different structs representing the same information,
including in components indepent of the PeerConnection api. One of the
structs is therefore renamed and promoted to api, with its own header
file.

Since cl https://webrtc-review.googlesource.com/74020, there's a
version of the PeerConnectionInterface::SetBitrate method that accepts
the new struct. The old struct will be deleted in two weeks time, see
cl https://webrtc-review.googlesource.com/75620.

Inside the structs, the member
PeerConnectionInterface::BitrateParameters::current_bitrate_bps is
replaced by BitrateSettings::start_bitrate_bps, matching current
implementation. It is intended for letting the application override
initial bitrate settings for video encoders, affecting the size of the
initial key frame, as well as the initial state of the bandwidth
estimator.