chore(cognito-identitypool): make role attachment resource public (#34234)

### Issue # (if applicable)

N/A

### Reason for this change

In order to allow for more flexibility when configuring Identity Pool resources, one may need to override attributes in the role attachment. To make this process easier, we should be able to access this resource the same way as we would access the L2 resource.

### Description of changes

```ts
  // private readonly roleAttachment: IdentityPoolRoleAttachment;
  public readonly roleAttachment: IdentityPoolRoleAttachment;
```

### Describe any new or updated permissions being added

N/A

### Description of how you validated changes

`yarn build && yarn test aws-cognito-identitypool`

### 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: Leo10Gama

packages/aws-cdk-lib/aws-cognito-identitypool/lib/identitypool.ts

@@ -440,7 +440,7 @@ export class IdentityPool extends Resource implements IIdentityPool {
   /**
    * Role Provider for the default Role for authenticated users
    */
-  private readonly roleAttachment: IdentityPoolRoleAttachment;
+  public readonly roleAttachment: CfnIdentityPoolRoleAttachment;
 
   /**
    * List of Identity Providers added in constructor for use with property overrides
@@ -500,7 +500,7 @@ export class IdentityPool extends Resource implements IIdentityPool {
       authenticatedRole: this.authenticatedRole,
       unauthenticatedRole: this.unauthenticatedRole,
       roleMappings: props.roleMappings,
-    });
+    }).resource;
 
     Array.isArray(this.roleAttachment);
   }
@@ -589,6 +589,11 @@ class IdentityPoolRoleAttachment extends Resource implements IIdentityPoolRoleAt
    */
   public readonly identityPoolId: string;
 
+  /**
+   * The Identity Pool Role Attachment CFN resource.
+   */
+  public readonly resource: CfnIdentityPoolRoleAttachment;
+
   constructor(scope: Construct, id: string, props: IdentityPoolRoleAttachmentProps) {
     super(scope, id);
     // Enhanced CDK Analytics Telemetry
@@ -604,7 +609,7 @@ class IdentityPoolRoleAttachment extends Resource implements IIdentityPoolRoleAt
     if (mappings) {
       roleMappings = this.configureRoleMappings(...mappings);
     }
-    new CfnIdentityPoolRoleAttachment(this, 'Resource', {
+    this.resource = new CfnIdentityPoolRoleAttachment(this, 'Resource', {
       identityPoolId: this.identityPoolId,
       roles,
       roleMappings,