Establish best practices for use of the x_ fields

[david-waltermire]

The QWG needs to determine what the best practices are for use of the x_ fields. This may involve providing a means to get a JSON schema for the field and/or any associated documentation provided by the declaring CNA.

Schema validation could be supported as follows.

{ "x_foo": {
  "$schema": "schema URL",
  .. data
  }
}

Possible options need to be discussed to come up with a way forward. Please share thoughts in the discussion below.