Support classification name filtering when creating a workers process

Refs https://gitlab.com/teklia/requests/-/issues/864

The socface project needs filtering on class names when creating new process.

We will NOT support classification worker version or classification confidence as it's not needed to start these processes and would complexify the development.

• CreateElementsWorkflow endpoint must support a new class_id optional filter
• the Process model must be extended to have a class_id nullable string
• Process.list_elements() must filter elements having a classification with that class ID