Add "before_validate" hook to document.py (#6112)

Reference: https://discuss.erpnext.com/t/override-default-validation/20622/5

The addition of a `before_validate` hook makes it possible to override validation methods. This feature has been requested in numerous threads. See:
https://discuss.erpnext.com/t/override-controllers-events/25723/2
https://discuss.erpnext.com/t/is-method-overriding-in-frappe-possible/9225/19
https://discuss.erpnext.com/t/how-to-override-method-in-frappe/28786/8

frappe/model/document.py

@@ -875,9 +875,11 @@ class Document(BaseDocument):
 			return
 
 		if self._action=="save":
+			self.run_method("before_validate")
 			self.run_method("validate")
 			self.run_method("before_save")
 		elif self._action=="submit":
+			self.run_method("before_validate")
 			self.run_method("validate")
 			self.run_method("before_submit")
 		elif self._action=="cancel":