feat: implement advanced fulfillment features

[0x8000-0000]

Description

- Added 'calculate_available_methods' to 'FulfillmentService'.
- Updated 'CheckoutService' to support split shipments and 'available_methods' calculation.
- Fixed serialization issue for 'FulfillmentAvailableMethod'.
- Updated 'update_checkout' route to merge platform config from request body.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation update

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules: this change depends on feat: implement advanced fulfillment features samples#27