set a mark for agentized groups

[Harry Putnam]

emacs-26 gnus 5.13

If there are even people left who use gnus-agent... I've just went
back to using it after a very long hiatus.

I wondered if anyone knows of a way to to mark groups in the group
buffer that indicates they are agentized.

I know about agentizing servers and when I have a server agentized of
course I know which groups.

But in some groups I didn't want the entire server agentized so now
I'd like to know at a glace which are under the agent in that server.

Another step along this same vein is a mark that I would know meant
that the whole group was downloaded or at least as much of it as I
want.