Making properties writeable at creation only

A "writeable only at creation" property is a property whose value can only be specified in a POST and not in any PATCH. This is used for fields that, once set, cannot be modified.

To specify a property is writeable only at creation, set the x-gw-extensions object's create-only property to true:

"x-gw-extensions": {
  "create-only": true
}

For example, suppose the CustomEntity_Ext entity has an ExpirationDate field. This field can be set on a POST, but not in any PATCH. The property declaration would be:

"definitions": {
  "CustomEntityExt": {
    ...
    "properties": {
      ...
	"expirationDate": {
	  "x-gw-extensions": {
	    "create-only": true
	  }
      }
    }
  }
}