Add WebSocket messages for beatmap data and user activity

[tsunyoku]

Warning

To any onlookers, this API is still rapidly changing. Please expect further breaking changes and do not use this PR to flood requests for what data should be available.

RFC.

Example beatmap message

{
    "beatmap_id": 1614228,
    "beatmap_set_id": 760694,
    "beatmap_hash": "1cf9f752f9c92dc5b8a2138d0c2e1653",
    "metadata": {
        "artist": "Loki",
        "artist_unicode": "Loki",
        "title": "A New King Is Born",
        "title_unicode": "A New King Is Born",
        "author": "DTM9 Nowa",
        "source": "osu!",
        "tags": "Mapping with rewards MWR The Fall of a Hero symphonic metal",
        "user_tags": []
    },
    "difficulty": {
        "approach_rate": 8.5,
        "circle_size": 3.5,
        "overall_difficulty": 6.0,
        "drain_rate": 5.0
    },
    "difficulty_name": "Hyper",
    "ruleset_id": 0,
    "bpm": 190.0,
    "star_rating": 4.05,
    "status": 1,
    "type": "BeatmapMessage"
}

Example user activity message

{"status":"ChoosingBeatmap","type":"UserActivityMessage"}

[tsunyoku]

We might want this enum (and any future cases) to serialize as a string rather than an int, depending on what future documentation for the WebSocket may look like once it reaches a mature state.

Indifferent, would appreciate another opinion.