AWS::FSx::Volume OnTapConfiguration SizeInGigabytes property to match AWS::FSx::FileSystemStorageCapacity property

### Name of the resource

AWS::FSx::FileSystem

### Resource name

AWS::FSx::Volume

### Description

When building a CFN template that creates an FSx ONTAP filesystem and volume, I have to have two parameters to specify the `AWS::FSx::FileSystem` `StorageCapacity` (in GB) and then a second to specify the `AWS::FSx::Volume` `OntapConfiguration` `SizeInMegabytes` property (in MB).

For scenarios where I am only building one SVM and one FSx Volume, having to specify the same parameter twice with one multiplied by 1000 can feel confusing to consumers of the CFN template.

I am requesting an optional property for the `AWS::FSx::Volume.OntapConfiguration` called `SizeInGigabytes` which cannot be specified with `SizeInMegabytes` and sets the same value but in GB instead of MB.

Thanks
-Alex

### Other Details

_No response_

Start by reviewing the AWS::FSx::Volume resource schema, especially OntapConfiguration and the existing SizeInMegabytes property. Confirm how optional properties are validated and how AWS::FSx::FileSystem StorageCapacity is represented; done means SizeInGigabytes is supported in GB, cannot be combined with SizeInMegabytes, and the resource behavior is covered by the project’s relevant validation or coverage checks.

Written by the indexing model from the issue text.