(neptune-alpha): Fix construct to dynamically update supported engine versions

### Describe the feature

The construct does not automatically update the supported engine versions and supported instance families. It needs to be altered to fetch the current supported engine versions via: [DescribeDBEngineVersionsCommand](https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/client/neptune/command/DescribeDBEngineVersionsCommand)

And the latest set of supported instance types/sizes via: [DescribeOrderableDBInstanceOptionsCommand](https://docs.aws.amazon.com/AWSJavaScriptSDK/v3/latest/client/neptune/command/DescribeOrderableDBInstanceOptionsCommand)

### Use Case

Provide users the ability to configure latest engine versions for a cluster along with the full list of supported instance types when using the neptune L2 construct (alpha).

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.178.2

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

N/A

Start with the neptune L2 construct and read the linked DescribeDBEngineVersionsCommand and DescribeOrderableDBInstanceOptionsCommand documentation. Done means the construct dynamically obtains current supported engine versions and the full latest set of supported instance types and sizes.

Written by the indexing model from the issue text.