Exposing the micropub client information

[aciccarello]

This probably needs to be refined, but I would like to show what micropub client was used to create a post. My inspiration for this is Aaron's site which shows a message like "Posted in /photos using quill.p3k.io" at the bottom of his posts.

For my site, I would like to see something returned in the properties passed to the file formatter. I assume this would need to be returned by the auth/introspect endpoint.

I had a brief discussion with Aaron in #indieweb-dev chat about how he implemented it.