Add cost based on GPU usage

[vickmp]

Description Based on #6581, add a cost for GPU usage in the same way as currently exists for memory, disk or CPU.

Use case Being able to add a cost for GPU usage opens up the possibility of being able to calculate costs and provide GPU services to different end users in a simple and convenient way.

Additional context It's important to note that not all VMs have GPUs (as is the case with CPU, memory and disk), so this should be taken into account when processing VM information.

Progress Status

• [ ] Code committed
• [ ] Testing - QA
• [ ] Documentation (Release notes - resolved issues, compatibility, known issues)