Lexicons, add optional title to external

I really like this change. I think it'll make records way easier to
understand in the repo, and more useful if the external site goes down.

Also allows shortcut querying if you don't need more data than the title

lexicons/my/spoor/content/external.json

@@ -9,6 +9,14 @@
         "type": "object",
         "required": [ "source", "queryable" ],
         "properties": {
+          "titles": {
+            "type": "array",
+            "minLength": 1,
+            "items": {
+              "type": "ref",
+              "ref": "my.spoor.content.defs#title"
+            }
+          },
           "source": {
             "type": "string",
             "description": "An nsid for a specific data source. The domain authority governs how to process the queryable",