add Basic support for Smooth Streaming protocol(#8118)

youtube_dl/downloader/__init__.py

@@ -7,6 +7,7 @@ from .http import HttpFD
 from .rtmp import RtmpFD
 from .dash import DashSegmentsFD
 from .rtsp import RtspFD
+from .ism import IsmFD
 from .external import (
     get_external_downloader,
     FFmpegFD,
@@ -24,6 +25,7 @@ PROTOCOL_MAP = {
     'rtsp': RtspFD,
     'f4m': F4mFD,
     'http_dash_segments': DashSegmentsFD,
+    'ism': IsmFD,
 }