Find additional information for RelationLink

[bruderj15]

RelationLink describes the relation of one resource to another via a link.

Some content-types like application/vnd.collection+json (#15) or application/prs.hal-forms+json (#17) provide predefined attributes to describe the endpoint behind the referencing link in more detail.

For example, part of a application/prs.hal-forms+json response may look like:

{
  "_templates" : {
    "default" : {
      "contentType" : "application/json",
      "href": "/api/resource/1",
      "method": "PUT",
      "properties": [
         {
           "name": "name",
           "required": true,
           "type": "text"
         }
      ]
    }
  }
}

Information like contentType and method are easily accessible for us.
There is much more information around.

Can we utilize combinators Summary and Description? (#23)
Can we add information regarding a POST-Requests body?
Can we add information regarding Query-Params?
And so on...

This is just about gathering the information in HasRelationLink and storing it in RelationLink.
Concrete usage in explicit resource-types is obviously handled by the MimeRender instances for the corresponding content-types.

Gather all possible information we can find and make separate issues for each of them.