Need to mark codec-related definitions for export

[alvestrand]

The following definitions need to be exported in order to land w3c/webrtc-encoded-transform#186 with the right links:

• final steps to create an offer
• final steps to create an answer
• list of implemented receive codecs (which should also be marked as an RTCRtpReceiver property, to match the list of implemented send codecs anchor)

The WEBRTC-PC spec also refers to "list of codecs which the user agent is currently prepared to receive" and "list of codecs which the user agent is currently capable of sending"; these terms are not tagged or defined further. We might want to add definitions for these.