Use oEmbed for YouTube Shorts (#15025)

It seems that YouTube Short previews do not work in some
regions, but the oEmbed information for those areas is still
valid.

This causes YouTube Shorts to always use (only) the oEmbed
endpoint which is a minor regression for regions where the URL
preview was already working -- some of the additional video
metadata is lost. It is not likely that clients are using this today
and it is more beneficial to have a limited preview working everywhere
than unused metadata in the Open Graph response.

changelog.d/15025.misc

@@ -0,0 +1 @@
+Use oEmbed to generate URL previews for YouTube Shorts.

synapse/res/providers.json

@@ -11,5 +11,18 @@
                 "url": "https://publish.twitter.com/oembed"
             }
         ]
+    },
+    {
+        "provider_name": "YouTube Shorts",
+        "provider_url": "http://www.youtube.com/",
+        "endpoints": [
+            {
+                "schemes": [
+                    "https://youtube.com/shorts/*",
+                    "https://*.youtube.com/shorts/*"
+                ],
+                "url": "https://www.youtube.com/oembed"
+            }
+        ]
     }
 ]