Support custom Devfile via the workspace creation GraphQL API without committing to repo

Allow users to provide a custom Devfile via the Workspace API during creation, without requiring it to be committed to the repository. This enables more flexible and dynamic workspace setups, especially for ad-hoc configurations.

Proposal

• Extend the Workspace creation API to accept a custom devfile field
  • Open Question: What's the format of this field?
• If provided, this Devfile should override any Devfile in the repository (if any).
• Ensure validation and fallback behaviour is clear (e.g., use repo Devfile if none provided).