Add indicator if schema is managed and mutable to Schema REST api.

The current response to /collection/schema does not include any hint if the schema can be modified using the schema rest API.

It would be great to have this information available via an additional attribute in the response and/or as separate entry point

collection/schema/managed?wt=json

{
  "responseHeader":{
    "status":0,
    "QTime":1
  },
  "managed":true
}