Implement delivery creation with auto-incrementing IDs

[rexx010]

Closes #20

Description

This PR implements the create_delivery function for the delivery smart contract, introducing deterministic ID generation and persistent storage of delivery records.

Changes

• Storage Updates: Added DeliveryCounter to DataKey enum and expanded the DeliveryRecord struct to include delivery_id and created_at.
• Delivery Creation Logic: Implemented create_delivery to:
  • Enforce sender authorization.
  • Auto-increment a persistent ID counter.
  • Store the new delivery record with a Pending status and extended TTL.
  • Emit the delivery_created event.
• Testing: Added unit tests verifying successful creation, correct state initialization, and persistent, incrementing IDs across multiple calls.

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