chore(custom-resources): add @experimental decorator to CustomResourceConfig (#31177)

### Reason for this change

Follow the style we use for it here, for consistency: https://github.com/aws/aws-cdk/blob/76e7af6f232655fed60619e2a5f9c629f1a46d1c/packages/aws-cdk/lib/api/plugin/plugin.ts#L125

### Description of changes

Added the decorator.

### Description of how you validated changes

Comment only change, no tests modified or run. 

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: comcalvi

packages/aws-cdk-lib/custom-resources/lib/custom-resource-config/custom-resource-config.ts

@@ -12,6 +12,7 @@ export const CUSTOM_RESOURCE_SINGLETON_LOG_RETENTION = 'aws:cdk:is-custom-resour
 
 /**
  * Manages AWS-vended Custom Resources
+ *
  * This feature is currently experimental.
  */
 export class CustomResourceConfig {
@@ -42,6 +43,8 @@ export class CustomResourceConfig {
 
 /**
  * Manages log retention for AWS-vended custom resources.
+ *
+ * This feature is currently experimental.
  */
 export class CustomResourceLogRetention implements IAspect {
   private readonly logRetention: logs.RetentionDays;
@@ -89,6 +92,8 @@ export class CustomResourceLogRetention implements IAspect {
 
 /**
  * Manages removal policy for AWS-vended custom resources.
+ *
+ * This feature is currently experimental.
  */
 export class CustomResourceRemovalPolicy implements IAspect {
   private readonly removalPolicy: RemovalPolicy;