…ret (aws#21262)

Consumers of the `cognito.UserPoolClient` construct currently have a way to retrieve useful information and pass that information to other pieces of infrastructure (e.g. Lambdas, ECS, Parameter Store, CfnOutput), e.g. the Client ID, which is useful for certain operations against the Cognito user pool that require a Cognito client.

However, if consumers decide to configure the Client, with a pre-generated (random) Client Secret for security reasons, by passing the `generateSecret: true` prop to `cognito.UserPoolClient`, most operations to the Cognito UserPool now require the Client ID AND Client Secret, otherwise they generate the `Unable to verify secret hash for client <client-id>` error (see [AWS support article](https://aws.amazon.com/premiumsupport/knowledge-center/cognito-unable-to-verify-secret-hash/) for more info).

Currently, the construct exposes no way to retrieve the Client Secret, forcing consumers to work around this by using custom resources or custom AWS SDK calls to retrieve it.

This PR addresses that, by exposing a new getter method of the `UserPoolClient` class, allowing consumers to read it on demand and pass it to other pieces of infrastructure, using code like:

```ts
const userPoolClient = new cognito.UserPoolClient(this, 'UserPoolClient', {
  userPool,
  generateSecret: true,
});

// Allows you to pass the generated secret to other pieces of infrastructure
const secret = userPoolClient.userPoolClientSecret;
```

Closes aws#7225

----

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