Implement assign_driver for delivery contract

[rexx010]

Closes #21

Description

This PR introduces the secure assign_driver function in the delivery_contract to handle driver assignments for deliveries.

Key Changes

• Access Control: Enforced require_auth(), restricting callers to either an Admin or the Driver themselves (self-assignment).
• State Machine: Validated that assignments can only occur when the delivery is in a Pending state, transitioning it to Active.
• Storage & Events: Extended storage TTL (~30 days) and emitted the driver_assigned event upon successful assignment.
• Testing: Added comprehensive unit tests covering successful admin/driver assignments, unauthorized rejections, and invalid state panics.
• Shared Types: Fixed event symbol length limits in the shared_types crate.

Testing

• cargo test -p delivery_contract passes successfully.

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