Implemented lower() and upper() functions

For use in functional expressions: [?lower(foo) == 'bar']

Author: cs-christopher-carsey

jmespath/functions.py

@@ -177,6 +177,14 @@ def _subtype_check(self, current, allowed_subtypes, types, function_name):
     @signature({'types': ['number']})
     def _func_abs(self, arg):
         return abs(arg)
+    
+    @signature({'types': ['string']})
+    def _func_lower(self, arg):
+        return arg.lower()
+    
+    @signature({'types': ['string']})
+    def _func_upper(self, arg):
+        return arg.upper()
 
     @signature({'types': ['array-number']})
     def _func_avg(self, arg):