Add a singleton global data model

Right now it's impossible to add a one-off global data model. An example where this could be useful is something like a highlight article. You could create a model that references an article. This model should only have one entry. Right now you could add an infinite amount of entries to that model. The only workaround is to limit the entries with a validation hook, which is not ideal.

This is a basic feature which almost any other CMS has, e.g. payload CMS: https://payloadcms.com/docs/configuration/globals