Add support for validation message callback

- Adds a type check for message just before attempting to process validation params on it
- Resolves the callback if it was one using the validation rule for the 'this' context

app/code/Magento/Ui/view/base/web/js/lib/validation/validator.js

@@ -48,6 +48,10 @@ define([
                 params :
                 [params];
 
+            if (typeof message === 'function') {
+                message = message.call(rule);
+            }
+
             message = params.reduce(function (msg, param, idx) {
                 return msg.replace(new RegExp('\\{' + idx + '\\}', 'g'), param);
             }, message);