Lavalink

This release adds support for Lavalink server v4 (with REST API).

For a full changelog, you can [view the commits here](https://github.com/devoxin/Lavalink.py/compare/4.0.7...5.0.0)

Fixes
- Fix bad `endTime` check.
- Additionally, raise minimum `endTime` duration to `1`.

Additions
- Add `LoadResult.selected_track` convenience property.
- Add `Client.get_source()` convenience method.

Changes
- Raise original error in `utils.timestamp_to_millis`.
- Slight improvement to `DefaultPlayer.play` documentation to clarify the difference between it and `BasePlayer.play_track`.

Fixes
- Fixed `DefaultPlayer.play` not throwing if the provided track does not have a `track` str, and it is not an instance of `DeferredAudioTrack`.

Additions
- Add return type to `utfm_codec.read_utfm`.
- Add return type to `utils.encode_track`.
- Improved typedefs of `Stats` object.
- Improved typedefs of `Penalty` object.
- Improved typedefs of `Node`.
- Improved typedefs of `NodeManager`.
- Improved typedefs of `PlayerManager`.
- Improved typedefs of `QueueEndEvent`.
- Improved typedefs of `PlayerUpdateEvent`.
- Improved typedefs of `WebSocketClosedEvent`.
- Improved typedefs of track events.
- Improved typedefs of node events.

- Added `ping` field to `PlayerUpdateEvent` after it somehow got removed.
- Added `Node` type def to `BasePlayer.node`, `BasePlayer.change_node`, and `DefaultPlayer.__init__`.
- Changed `Equalizer.update` to also accept `float` for the `gain` value.

Fixes
- Use asyncio.gather over asyncio.wait

Additions
- Add rudimentary track encoding support

Changes
- Don't default to -1 if frame stats missing.

Fixes
- Fix `DefaultPlayer._last_update` being incorrectly reset in `play()` leading to brief periods where `position` would read incorrectly between tracks.