Playlist fixes, add YoutubeDl::get_stream function (#279)

* Fix(ytdl): Return all results when querying a URL

* Publicly export `songbird::input::metadata::ytdl::Output`

Closes: https://github.com/serenity-rs/songbird/issues/277

* Make `YoutubeDl::query` public

* Abstract getting streams from YoutubeDl into functions `get_stream` and `get_streams`

* fmt, remove get_streams

* Fix doc comment

* fixup doc comments, export `metadata::ytdl::Output` as `YoutubeDlOutput`

* fmt

* export metadata

* fixup! fixup doc comments, export `metadata::ytdl::Output` as `YoutubeDlOutput`

src/input/metadata/ytdl.rs

@@ -1,28 +1,49 @@
+//! `YoutubeDl` track metadata.
+
 use super::AuxMetadata;
 use crate::constants::SAMPLE_RATE_RAW;
 use serde::{Deserialize, Serialize};
 use std::{collections::HashMap, time::Duration};
 
+/// Information returned by yt-dlp about a URL.
+///
+/// Returned by [`crate::input::YoutubeDl::query`].
 #[derive(Deserialize, Serialize, Debug)]
 pub struct Output {
+    /// The main artist.
     pub artist: Option<String>,
+    /// The album name.
     pub album: Option<String>,
+    /// The channel name.
     pub channel: Option<String>,
+    /// The duration of the stream in seconds.
     pub duration: Option<f64>,
+    /// The size of the stream.
     pub filesize: Option<u64>,
+    /// Required HTTP headers to fetch the track stream.
     pub http_headers: Option<HashMap<String, String>>,
+    /// Release date of this track.
     pub release_date: Option<String>,
+    /// The thumbnail URL for this track.
     pub thumbnail: Option<String>,
+    /// The title of this track.
     pub title: Option<String>,
+    /// The track name.
     pub track: Option<String>,
+    /// The date this track was uploaded on.
     pub upload_date: Option<String>,
+    /// The name of the uploader.
     pub uploader: Option<String>,
+    /// The stream URL.
     pub url: String,
+    /// The URL of the public-facing webpage for this track.
     pub webpage_url: Option<String>,
+    /// The stream protocol.
     pub protocol: Option<String>,
 }
 
 impl Output {
+    /// Requests auxiliary metadata which can be accessed without parsing the file.
     pub fn as_aux_metadata(&self) -> AuxMetadata {
         let album = self.album.clone();
         let track = self.track.clone();