Content, collapse source/queryable, add tvdbv4

Removed the source field, and made queryable be a union where the type
will reveal how to query, and the data to query will be contained
within. (This is much smarter)...

Changed overrides to be a simple empty object (which should be fine in
the spec). This reflects what it should actually be.

Finally explicitly added the tvdb api v4 option.

my/spoor/content/external.json

@@ -7,8 +7,15 @@
       "key": "nsid",
       "record": {
         "type": "object",
-        "required": [ "source", "queryable" ],
+        "required": [ "queryable" ],
         "properties": {
+          "queryable": {
+            "type": "union",
+            "description": "All the data needed to query the content from the source",
+            "refs": [
+              "#tvdbv4"
+            ]
+          },
           "titles": {
             "type": "array",
             "minLength": 1,
@@ -17,26 +24,20 @@
               "ref": "my.spoor.content.title"
             }
           },
-          "source": {
-            "type": "string",
-            "description": "An nsid for a specific data source. The domain authority governs how to process the queryable",
-            "format": "nsid",
-            "knownValues": [
-              "my.spoor.content.external#tvdb"
-            ]
-          },
-          "queryable": {
-            "type": "union",
-            "description": "All the data needed to query the content from the source",
-            "refs": []
-          },
           "overrides": {
-            "type": "union",
+            "type": "object",
             "description": "User defined overrides for the returned content",
-            "refs": []
+            "properties": {}
           }
         }
       }
+    },
+    "tvdbv4": {
+      "type": "object",
+      "required": [ "id" ],
+      "properties": {
+        "id": { "type": "integer" }
+      }
     }
   }
 }