downloader: Add ability to download from an arbitrary playlist URL

This is a very limited feature, it can't handle multiple qualities.
You must use "source" only and it takes the first available rendition.

downloader/downloader/main.py

@@ -623,6 +623,8 @@ def main(channels, base_dir=".", qualities="source", metrics_port=8001, backdoor
 	for channel, important, type, url in channels:
 		if type == "twitch":
 			provider = TwitchProvider(channel, auth_token=twitch_auth_token)
+		elif type == "url":
+			provider = URLProvider(url)
 		else:
 			raise ValueError(f"Unknown type {type!r}")
 		manager = StreamsManager(provider, channel, base_dir, qualities, important=important)

downloader/downloader/providers.py

@@ -30,6 +30,27 @@ class Provider:
 		return hls_playlist.load(resp.text, base_uri=resp.url)
 
 
+class URLProvider(Provider):
+	"""Provider that takes an arbitrary master playlist URL.
+	Doesn't support multiple renditions, quality must be "source".
+	"""
+	def __init__(self, master_playlist_url):
+		self.master_playlist_url = master_playlist_url
+
+	def get_media_playlist_uris(self, qualities, session=None):
+		if qualities != ["source"]:
+			raise ValueError("Cannot provide non-source qualities")
+		if session is None:
+			session = InstrumentedSession()
+
+		resp = session.get(self.master_playlist_url, metric_name='url_master_playlist')
+		resp.raise_for_status()
+		master_playlist = hls_playlist.load(resp.text, base_uri=resp.url)
+
+		# Take the first variant
+		return {"source": master_playlist.playlists[0].uri}
+
+
 class TwitchProvider(Provider):
 	"""Provider that takes a twitch channel."""
 	# Twitch links expire after 24h, so roll workers at 20h