Adds an example demonstrating user, comment, and follower relationship
including:
- User management with profiles
- Comments (not really useful, just for showcasing)
- Follower/follozing relationships
- Ineractive CLI interface with CRUD operations
- Database migrations for the example schema
Introduces support for `#[georm(defaultable)]` attribute on entity
fields. When fields are marked as defaultable, generates companion
`<Entity>Default` structs where defaultable fields become `Option<T>`,
enabling easier entity creation when some fields have database defaults
or are auto-generated.
Key features:
- Generates `<Entity>Default` structs with optional defaultable fields
- Implements `Defaultable<Id, Entity>` trait with async `create` method
- Validates that `Option<T>` fields cannot be marked as defaultable
- Preserves field visibility in generated companion structs
- Only generates companion struct when defaultable fields are present
