neptune: (add storageType field to support the I/O optimized storage configuration)

### Describe the feature

AWS has announced the general availability of Amazon Neptune I/O-Optimized, a new configuration for Amazon Neptune Database that provides improved price performance and predictable pricing for customers with I/O-intensive applications. With Neptune I/O-Optimized, there are zero charges for read and write I/O operations—you only pay for your database instances and storage usage, making it easy to predict your database spend up front. Neptune I/O-Optimized offers up to 40% cost savings for I/O-intensive applications where I/O charges exceed 25% of the total Neptune database spend.


### Use Case

The support for feature seems to be already added for the rds module ([issue link](https://github.com/aws/aws-cdk/issues/25629), [PR link](https://github.com/aws/aws-cdk/pull/25704)), I would like to request it to be added for the neptune module as well.

One of our projects has heavy write loads, and I believe being able to switch to the I/O optimized configuration using the cdk module is going to be quite beneficial both in terms of speed and cost.

Unfortunately we can only perform production deployments through cdk, so we really need the module to be updated so we can perform the upgrade!

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.146.0

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

Linux

Start by reviewing the RDS implementation referenced in issue 25629 and PR 25704, then locate the corresponding Neptune module entry point. Confirm how the storageType field should represent Neptune I/O-Optimized configuration and identify the module's relevant tests. Done means CDK can configure this Neptune storage option for production deployments.

Written by the indexing model from the issue text.