chore(ec2): make applyCloudFormationInit() function public (#30907)

### Issue # (if applicable)

Closes #30863.

### Reason for this change

The issue is asking to make the function `applyCloudFormationInit()` public from private.

Also we have this function being public in https://github.com/aws/aws-cdk/blob/ffd9d9ce510a4c820b1437cce93c4772cd7c14c0/packages/aws-cdk-lib/aws-autoscaling/lib/auto-scaling-group.ts#L1650 

### Description of changes

Make the function public

### Description of how you validated changes
build passed

### 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: xazhao

packages/aws-cdk-lib/aws-ec2/lib/instance.ts

@@ -630,7 +630,7 @@ export class Instance extends Resource implements IInstance {
    * - Add commands to the instance UserData to run `cfn-init` and `cfn-signal`.
    * - Update the instance's CreationPolicy to wait for the `cfn-signal` commands.
    */
-  private applyCloudFormationInit(init: CloudFormationInit, options: ApplyCloudFormationInitOptions = {}) {
+  public applyCloudFormationInit(init: CloudFormationInit, options: ApplyCloudFormationInitOptions = {}) {
     init.attach(this.instance, {
       platform: this.osType,
       instanceRole: this.role,