AWS::MSK::ServerlessCluster & AWS::MSK::Cluster - Add bootstrapServers as an output

### Name of the resource

Other

### Resource name

AWS::MSK::ServerlessCluster & AWS::MSK::Cluster

### Description

Creating an `AWS::MSK::ServerlessCluster` in cloudformation can be done like:
```yaml
ServerlessCluster:
Type: AWS::MSK::ServerlessCluster
Properties:
ClusterName: !Sub "${AWS::StackName}-cluster"
ClientAuthentication:
Sasl:
Iam:
Enabled: true
VpcConfigs:
- SecurityGroups:
- !Ref securityGroup
SubnetIds:
- !Ref subnetA
- !Ref subnetB
- !Ref subnetC
```

If you also want to define a `AWS::KafkaConnect::Connector` on that cluster in the same template, the `BootstrapServers` parameter is required like this:
```yaml
MyKafkaConnectConnector:
Type: AWS::KafkaConnect::Connector
Properties:
KafkaCluster:
ApacheKafkaCluster:
BootstrapServers: **BootstrapServers**
Vpc:
SecurityGroups:
- !Ref securityGroup
Subnets:
- !Ref subnetA
- !Ref subnetB
- !Ref subnetC
```

I would propose a new output on the `AWS::MSK::ServerlessCluster` & `AWS::MSK::Cluster` resource called `bootstrapServers` that can be retreived like: !GetAtt ServerlessCluster.bootstrapServers

### Other Details

_No response_

Start by locating the AWS::MSK::ServerlessCluster and AWS::MSK::Cluster resource definitions and how GetAtt outputs are exposed. Review the AWS::KafkaConnect::Connector KafkaCluster.ApacheKafkaCluster.BootstrapServers requirement and compare the expected CloudFormation behavior. Done means both MSK resources expose bootstrapServers through !GetAtt for use by the connector.

Written by the indexing model from the issue text.