Protocol allows to specify targets by using actors URI or list of followers URI

In some examples of the ActivityPub protocol the to attribute contains an URI to an actor:

"to": ["https://chatty.example/ben/"],

and the ActivityStream vocabulary defines that the to attribute might be either a link or an object. So in the first implementation we considered that the to attribute would be a list of references or object to actors.

However, some examples of the ActivityPub protocol contains things like that:

"to": ["https://social.example/alyssa/followers/",
                   "https://www.w3.org/ns/activitystreams#Public"],

besides the "public" which is a special case, the URI to followers is actually typed to be an object or reference to a collections of actors.
So we have now a typing issue we should solve to be able to handle special cases like this one.