Expose letter attachment details via template API

[LeoCie]

We have steps in our pipeline which check the state of the templates in Notify against versions we have saved in our codebase.

As the current notify API does not expose any attachment details for letters this pipeline would currently not be able to identify if someone had updated the attachments (e.g. unknowingly, or a bad actor). I think the most useful would be the attachment version or ID (if this changes when attachments are updated), as the content itself is PDF and probably not useful to track.