fix(core): introduce DojoStore trait to handle some specific storage cases

[remybar]

Description

In the current Dojo version, Option<T> and custom enums are not correctly stored in the world storage, because the first variant id is 0 and cannot be differenciated from an uninitialized variant.

To fix that, this PR introduces the DojoStore trait.

• Every data structure implement the DojoStore trait which, by default, encapsulates Serde calls.
• The implementation of the DojoStore trait for custom enums and Option<T> handles the variant index incrementing and returns Default::default() as default value during deserialization if the variant index is 0. That means, each custom enums must implement the Default trait.

As these changes may break the world storage, we also introduce the DojoLegacyStorage derive attribute. This attribute must be used on models for which we want to keep the current storage behavior. So, by default, model (de)serialization is done throughDojoStore. If DojoLegacyStorage is set, model (de)serialization is done through Serde.

Tests

• Yes
• No, because they aren't needed
• No, because I need help

Added to documentation?

• README.md
• Dojo Book
• No documentation needed

Checklist

• I've formatted my code (scripts/prettier.sh, scripts/rust_fmt.sh, scripts/cairo_fmt.sh)
• I've linted my code (scripts/clippy.sh, scripts/docs.sh)
• I've commented my code
• I've requested a review after addressing the comments