RTCStatsType value for datachannel stata

The current RTCStatsType is defined like this:

enum RTCStatsType {
    "inboundrtp",
    "outboundrtp",
    "session",
    "track",
    "transport",
    "candidatepair",
    "localcandidate",
    "remotecandidate"
};

There are no values for the datachannel stats.

To make this symmetric with RTP, I suggest we add "inbounddata" and "outbounddata".

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