Move RtcRtpEncodingParameters into separate module

[turtletongue]

Part of #210

Synopsis

RtcRtpEncodingParameters is defined in the native API module with unrelated logic.

Solution

RtcRtpEncodingParameters should be moved inside a new rtc_rtp_encoding_parameters submodule.

Checklist

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[turtletongue]

FCM

Move `RtcRtpEncodingParameters` into a submodule in `medea-flutter-webrtc-native` crate (#210, #223)