[Feature request] UDAs specific for annotations

# Feature request

I think UDAs are a great feature, so I think we should be able to define UDAs also for annotations.

For instance, we could define an UDA `type` that would enable categorizing the different annotations, something like "progress", "info", "note", whatever...

An other example would be, in a multiple user environment, to add an `author` UDA, so that multiple users could add annotations.

Finally, a `fingerprint` UDA would also be helpful, with the ability to sign annotations using PGP.

Those examples are all I came up with, but I'm sure there could be plenty of other use.

Thank you for your attention.