fix(types): allow any value for AddFields

Fix #12096

Author: vkarpov15

test/types/PipelineStage.test.ts

@@ -415,3 +415,11 @@ const stages4: PipelineStage[] = [
     }
   }
 ];
+
+(function gh12096() {
+  const data: PipelineStage.AddFields = {
+    $addFields: {
+      name: { $meta: 'Bill' }
+    }
+  };
+})();

types/expressions.d.ts

@@ -2449,7 +2449,7 @@ declare module 'mongoose' {
     FunctionExpression |
     ObjectIdExpression |
     ConditionalExpressionOperator |
-    Expression.Let;
+    any;
 
   export type ObjectIdExpression =
     TypeExpressionOperatorReturningObjectId;