(neptune-alpha): Add PubliclyAccessible option to DatabaseCluster options

### Describe the feature

Recently, AWS Neptune released v1.4.6.0 which includes a feature for instances to have a public IP assigned. This new feature is currently unavailable in CloudFormation and CDK.

https://docs.aws.amazon.com/neptune/latest/userguide/neptune-public-endpoints.html

### Use Case

We currently use a convoluted setup with a Network Load Balancer to be able to access the cluster externally, while this option would make it very straightforward.

### Proposed Solution

Add `PubliclyAccessible` as a L1 construct option and include support for it in the `neptune-alpha` package.

### Other Information

The documentation does state that support is included in CloudFormation, however I am not able to find the relevant option and setting it with `addPropertyOverride` leads to an error (extraneous key [PubliclyAccessible] is not permitted).

### Acknowledgements

- [x] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

2.218.0

### AWS CDK CLI version

2.1029.3

### Environment details (OS name and version, etc.)

N/A

Start by locating the neptune-alpha package and its DatabaseCluster L1 construct and option definitions. Check how existing CloudFormation properties are represented, then verify that PubliclyAccessible is accepted and exposed as a DatabaseCluster option without the extraneous-key error.

Written by the indexing model from the issue text.