fix: improve type annotation for CustomResource properties

My team just went on quite the adventure trying to figure out why
JSON `false` values where getting converted to `"false"`. It would have
e been really nice if cdk's typing could have prevented that bug in the
first place. (It would be even nicer if the underlying CloudFormation
bug were fixed, but this is better than nothing)

packages/aws-cdk-lib/core/lib/custom-resource.ts

@@ -4,6 +4,13 @@ import { RemovalPolicy } from './removal-policy';
 import { Resource } from './resource';
 import { Token } from './token';
 
+// Note that CloudFormation only supports the subset of JSON where the leaf nodes are strings.
+// See https://github.com/aws/aws-cdk-rfcs/issues/375 for details.
+type StringOnlyJsonValue =
+    | string
+    | { [x: string]: StringOnlyJsonValue }
+    | Array<StringOnlyJsonValue>;
+
 /**
  * Properties to provide a Lambda-backed custom resource
  */
@@ -57,10 +64,12 @@ export interface CustomResourceProps {
 
   /**
    * Properties to pass to the Lambda
+   * Note that CloudFormation only supports the subset of JSON where the leaf
+   * nodes are strings. See https://github.com/aws/aws-cdk-rfcs/issues/375 for details.
    *
    * @default - No properties.
    */
-  readonly properties?: { [key: string]: any };
+  readonly properties?: { [key: string]: StringOnlyJsonValue };
 
   /**
    * For custom resources, you can specify AWS::CloudFormation::CustomResource