Federation: Receive objects/targets/actors with only URLs set

Currently activities can only be received with some minimal serialized information

Object and target both require the rdf_type to deduce the local model storage

Object may also require necessary nested objects, for example as CircleMember requires circle and user relations, we must send:

"object": {
    "@type": "hd:circlemember",
    "@id": "https://distant.com/circle-members/1/",
    "user": {
        "@type": "foaf:user",
        "@id": user.urlid
    },
    "circle": {
        "@type": "hd:circle",
        "@id": "https://distant.com/circles/1/"
     }
},

To be fully compliant with ActivityStreams we should also be able to process objects/targets as URLs, e.g.

"object": "https://distant.com/circle-members/1/"

This will require us to fetch the resource from the URL and deduce that the rdf_type is hd:circlemember