(aws-msk-alpha): Cannot provide my own encryption key for SASL Scram authentication

### Describe the bug

ClientAuthentication.sasl properties currently allow you to provide an encryption key for the user secret. However, the construct does not leverage that key.

Current code for authentication configuration:

```
clientAuthentication: ClientAuthentication.sasl({
scram: true,
key: props.kmsEncryptionKeyForCredentialsSecret,
})
```

Then, add user:
` this.cluster.addUser(user)`

### Expected Behavior

It will use the encryption key I provide.

### Current Behavior

It does not use the encryption key, nor creates new one.
It throws an error:` Cannot create users if an authentication KMS key has not been created/provided.`

### Reproduction Steps

See above.

### Possible Solution

Initialize `this.saslScramAuthenticationKey` with the provided key, if set.

https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/aws-msk/lib/cluster.ts

### Additional Information/Context

_No response_

### CDK CLI Version

2.45

### Framework Version

_No response_

### Node.js Version

16.15.1

### OS

MacOS

### Language

Typescript

### Language Version

_No response_

### Other information

_No response_

Start in packages/@aws-cdk/aws-msk/lib/cluster.ts and follow ClientAuthentication.sasl through the addUser entry point. Check how saslScramAuthenticationKey is initialized and confirm that a provided key is retained; done means addUser no longer reports that an authentication KMS key is missing when one was supplied.

Written by the indexing model from the issue text.