Hyper-V Builder = Fixed VHD support for Generation 2

_This issue was originally opened by @thetaylormcrae as hashicorp/packer#8529. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

#### Feature Description

Allow the Hyper-V builder to create a fixed VHDX file instead of defaulting to dynamic.

#### Use Case(s)

New-VHD supports creating a fixed VHD and Gen 2 VMs support it. There are instances where a template needs to be Gen 2 and have a fixed hard drive for space constraints.