Adapt HookHandler to allow listing of routes

[ljucam]

Currently it’s not possible to list routes in a collection.

PUT /collection/resource1
PUT /collection/resource2/_hook/route
GET /collection/resource2 => HookHandler -> Forwarder
GET /collection => Resource storage => { "collection": [ "resource1" ] }

In order to be able to list routes in a collection, we have to adapt the HookHandler. This way a request to a parent collection will check, if routes exist and if so, list them properly. Therefor the HookHandler takes control of creating the listing, if a route - handled by the HookHandler – is present (not the Router), otherwise the Router takes control.

PUT /collection/resource1
PUT /collection/resource2/_hook/route
GET /collection/resource2 => HookHandler -> Forwarder
GET /collection => HookHandler -> Resource Storage + Route hook list => { "collection": [ "resource1", "resource2" ] }

Extend the configuration of Hooks for Routes with two new attributes ‘listable’ and ‘collection’.
listable => if true routes will be shown, otherwise not (current behavior)
collection => because it’s not visible by an URL, if it points to a collection or a resource, we need to specify if the rout is a collection (default) or a resource. This is necessary if the listable feature is used with for example the expand feature.

{ "destination": ..,          // as usual 
  "listable": false,          // new, allows to override the default
  "collection": false }       // new, indicates if route points to collection or resource. 

We also need a new constructor, which allows to set the default for the listable feature:
new HookHandler(...., true) // enable / disable listable as default