Cases for datetime types in extExprToValue (#568)

Signed-off-by: Adrian Palacios <accorell@amazon.com>

cedar-lean/CedarProto/Value.lean

@@ -43,6 +43,12 @@ private def extExprToValue (xfn : ExtFun) (args : List Expr) : Except String Val
   | .ip, [.lit (.string s)] => match Spec.Ext.IPAddr.ip s with
     | .some v => .ok $ .ext (.ipaddr v)
     | .none => .error s!"exprToValue: failed to parse ip {s}"
+  | .datetime, [.lit (.string s)] => match Spec.Ext.Datetime.parse s with
+    | .some v => .ok $ .ext (.datetime v)
+    | .none => .error s!"exprToValue: failed to parse datetime {s}"
+  | .duration, [.lit (.string s)] => match Spec.Ext.Datetime.Duration.parse s with
+    | .some v => .ok $ .ext (.duration v)
+    | .none => .error s!"exprToValue: failed to parse duration {s}"
   | _, _ => .error s!"exprToValue: unexpected extension value\n{repr (Expr.call xfn args)}"
 
 partial def exprToValue : Expr → Except String Value