feat: add metadata to shipping options endpoints

[bqst]

In Medusa v1, the /store/shipping-options endpoint exposed metadata for shipping options. This functionality was missing in v2, preventing developers from storing and retrieving custom key-value pairs on shipping options.

This PR restores metadata support for shipping options across both admin and store APIs.

Admin API:

• ✅ POST /admin/shipping-options now accepts metadata
• ✅ POST /admin/shipping-options/{id} now accepts metadata
• ✅ GET /admin/shipping-options & GET /admin/shipping-options/{id} already exposed metadata (unchanged)

Store API:

• ✅ GET /store/shipping-options now exposes metadata

Technical Updates:

• Added metadata field to AdminCreateShippingOption and AdminUpdateShippingOption validators
• Created missing AdminUpdateShippingOption.yaml schema
• Updated TypeScript interfaces in packages/core/types
• Updated store query config to include metadata by default
• Enhanced code examples with metadata usage

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[olivermrbl]

Overall LGTM, appreciate the contribution. I presume you tested this locally? Ideally, the PR includes a few integration tests.

As for the docs changes, I'll let @shahednasser decide what to do here. I believe these are usually added separately.

[bqst]

Thanks for the feedback! I've just added the corresponding integration tests.