Pluggable handler for collection assignment

[patrickmann]

/nocl /prd Graylog2/graylog-plugin-enterprise#11089

Description

Pluggable framework to support assignment of newly created entities to a set of collection. This feature is only available in Enterprise.

We take advantage of the major release to clean up entity creation request payload which breaks back-compat. An entity creation request now looks like this (e.g. for Stream). selected_collections is ignored in the Open edition.

{
   "entity":{
      "index_set_id":"65b7ba138cdb8c534a953fef",
      "description":"test",
      "title":"test",
      "remove_matches_from_default_stream":false
   },
   "share_request":{
      "selected_grantee_capabilities":{
         "grn::::search:684158906442150b2eefb78c":"own"
      },
      "selected_collections":{
         "grn::::search:684158906442150b2eefb78c"
      }
   }
}

Motivation and Context

See Graylog2/graylog-plugin-enterprise#11061