feat(lexicon): weaken link for playlists and plays

packages/lexicons/defs/sh/comet/v0/feed/play.json

···
         "type": "object",
         "required": ["subject", "createdAt"],
         "properties": {
-          "subject": { "type": "ref", "ref": "com.atproto.repo.strongRef" },
+          "subject": { "type": "string", "format": "at-uri" },
           "createdAt": { "type": "string", "format": "datetime" }
         }
       }

packages/lexicons/defs/sh/comet/v0/feed/playlist.json

···
           "tracks": {
             "type": "array",
             "minLength": 1,
-            "items": { "type": "ref", "ref": "com.atproto.repo.strongRef" }
+            "items": { "type": "string", "format": "at-uri" }
           },
           "image": {
             "type": "blob",