Convert WorkerVersion.model_usage to tri-state enum

Refs https://redmine.teklia.com/issues/5478

We need the field WorkerVersion.model_usage to behave like WorkerVersion.gpu_usage:

• use a tri-state enum (disabled | supported | required)
• convert existing values:
  • true => required
  • false => disabled

The WorkerVersionGPUUsage enum could be renamed to a more geenric FeatureUsage, but we coudl also simply create a dedicated enum with the same values.

These endpoints need to support the new flag:

• CreateDockerWorkerVersion
• CreateWorkerVersion
• ListWorkerVersions
• RetrieveWorkerVersion
• (Partial)UpdateWorkerVersion

You'll also need to check Process.build that uses the current model_usage boolean in pre-flight checks

For context, this is needed for training on datasets process: the worker version must support 3 states:

• required model when training/inference needs an existing model
• supported model when training can train from scracth OR fine tune an existing model
• disabled model when the worker has no support for any model whatsoever