[0005] Need capability advertisement/negotiation mechanism

[imax9000]

(Carried over from https://connect.iftas.org/groups/ozone-moderators/forum/discussion/ozone-moderation-history-api-proposal/)

“action” vs. “subject” distinction seems independent enough that it’s reasonable for some services to implement only one of them. To that end I think it would be great to have that explicitly advertised to the users in some way, so that the UI can adapt accordingly, instead of just showing no entries. (Checking for 404 doesn’t work, since it would be indistinguishable from many failure modes)