Add optional worker parameter to declare GPU usage

A new parameter set from the worker YAML configuration must be supported by the backend.

It's an enum named gpu_usage that can take 3 values:

• disabled (the default) : no GPU will be used, ever.
• supported : a GPU can be used
• required : a GPU must be used

This issue only adds parsing and updates unit tests: the value is then stored on the WorkerVersion configuration, ideally made available through a Python property on the model (like docker_command).