Enhance endpoint for doctypes with inbound relations

As a developer, my relations endpoint is now able to list inbound relations. The payload should look like:

{
  featureType: String,
  featureId: String,
  outbounds: [
     {
        featureType: String,
        featureId: String,
     }, ...
  inbounds: [
     {
        featureType: String,
        featureId: String,
     }, ...
  ]
}

AC

• A generic endpoint is available
• It can be used for doctypes at this stage
• It lists the following inbound relations
  • ActionFeature (User Action) :
    • filter rule
  • DocumentFeature (Doc type) :
    • container
    • parentDoc
    • accepted children if folderish
  • EventFeature (Event handlers) :
    • filter rule
  • WorkflowFeature :
    • filter rule
  • TemplateFeature ( Content template)
    • target doctype