Implement buy_ticket storage updates

[Utilitycoder]

Description

This PR implements storage updates for the buy_ticket function as specified in issue #198. It adds EventMetrics to track ticket sales and ensures UserTicket is correctly recorded for buyers.

Changes

• Types: Added EventMetrics struct and EventMetrics(u64) storage key.
• Interface: Added get_event_metrics and is_ticket_buyer to CrowdfundingTrait for better transparency and testing.
• Implementation: Updated buy_ticket to increment tickets_sold in EventMetrics and implemented the new getters.
• Testing: Added state logic tests in buy_ticket_test.rs to verify that metrics are updated and user tickets are recorded.

Verification

• cargo build --target wasm32-unknown-unknown --release succeeded.
• cargo test buy_ticket_test passed all 13 tests.

Closes #198

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