Ecto schema for a feature that can be granted by a plan.

Features are defined once and reused across plans. Each feature has a stable key, such as "export_csv" or "api_calls", and a type:

• "boolean" - the plan grants or denies a capability with "true" or "false".
• "limit" - the plan grants a numeric limit or "unlimited".

The value granted by each plan is stored in HawkEx.Billing.PlanFeature.