feat: shipment creation rate-limit exemption list for trusted suppliers

[shakurJJ]

Summary

Shipment creation is rate-limited per address (#363), which also throttles high-volume trusted suppliers. This proposes an admin-managed exemption list that bypasses the rate limit.

Proposed Behaviour

• Admin can add/remove supplier addresses to/from a rate-limit exemption list.
• create_shipment skips the rate-limit check for suppliers on the exemption list.
• Exempted suppliers remain subject to every other creation check (whitelist, blacklist, value bounds, etc).

Acceptance Criteria

• Admin can add/remove addresses from the exemption list and query membership
• Exempted suppliers bypass only the rate limit, nothing else
• Non-exempted suppliers are unaffected
• Tests cover exempt and non-exempt creation

[ifeanyikizito393]

@ifeanyikizito393 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer, I’m ready to own this issue. I understand the scope and can deliver a PR soon.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ifeanyikizito393 to this issue.