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
Currently, all methods declared in the Georm trait are available.
If a struct has an ID pointing towards another entity, the user can
create a get method to get the entity pointed at from the database
too (local one-to-one relationship).
I still need to implement remote one-to-one relationships (one-to-one
relationships when the ID of the remote object is not available
locally).
I still need to also test and debug one-to-many relationships (ID of
the remote entiies not available locally) and many-to-many
relationships (declared in a dedicated table).
For now, IDs in all cases are simple types recognized by SQLx that are
not arrays. Options are only supported when explicitely specified for
one-to-one relationships.
