Add 'optional' property for parameters in function specification

Author: Lettify

schemas/function.yaml

@@ -163,6 +163,10 @@ $defs:
           description: |
             The default value for this parameter, if none was given in the call to the function.
             This property automatically implicitly marks this parameter as optional.
+        optional:
+          type: boolean
+          default: false
+          description: If set to true, this parameter is optional.
 
   returns:
     type: object