[gstreamer] add a gst_discoverer API to obtain metadata

struct QGstDiscovererStreamInfo
{
    int streamNumber{};
    QString streamID;
    QGstTagListHandle tags;
    QGstCaps caps;
};

struct QGstDiscovererVideoInfo : QGstDiscovererStreamInfo
{
    QSize size;
    int bitDepth{};
    Fraction framerate{};
    Fraction pixelAspectRatio{};
    bool isInterlaced{};
    int bitrate{};
    int maxBitrate{};
    bool isImage{};
};

struct QGstDiscovererAudioInfo : QGstDiscovererStreamInfo
{
    int channels{};
    uint64_t channelMask{};
    int sampleRate{};
    int bitsPerSample{};
    int bitrate{};
    int maxBitrate{};
    QLocale::Language language{};
};

struct QGstDiscovererSubtitleInfo : QGstDiscovererStreamInfo
{
    QLocale::Language language{};
};

struct QGstDiscovererContainerInfo : QGstDiscovererStreamInfo
{
    QGstTagListHandle tags;
};

struct QGstDiscovererInfo
{
    bool isLive{};
    bool isSeekable{};
    std::optional<std::chrono::nanoseconds> duration;

    std::optional<QGstDiscovererContainerInfo> containerInfo;
    QGstTagListHandle tags;
    std::vector<QGstDiscovererVideoInfo> videoStreams;
    std::vector<QGstDiscovererAudioInfo> audioStreams;
    std::vector<QGstDiscovererSubtitleInfo> subtitleStreams;
    std::vector<QGstDiscovererContainerInfo> containerStreams;
};

----

QGstDiscoverer discoverer;
QGstDiscovererInfo result = discoverer.discover(aUrl);