[AWS::Neptune::DBCluster] - [Enhancement] - add storageType field to support the I/O optimized storage configuration

### Name of the resource

AWS::Neptune::DBCluster

### Resource name

_No response_

### Description

### 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 CFN support for attribute seems to be already added for the RDS and DocumentDB (https://github.com/aws-cloudformation/cloudformation-coverage-roadmap/issues/1870), I would like to request it to be added for Neptune 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, however, this depends on CFN supporting this attribute first.

Unfortunately we can only perform production deployments through CDK, so we would first need CFN to support the attribute before asking the CDK team to update the Neptune js module (CDK issue already created: https://github.com/aws/aws-cdk/issues/31078)

You can also refer to the --storage-type parameter in the DocDB CLI: https://docs.aws.amazon.com/neptune/latest/userguide/storage-types.html

### Other Details

_No response_