feat: Add flow control attributes for production payment processing (v1.4.0)

[jmarbutt]

Summary

• Adds [Throttle] attribute: QUEUES events at a rate limit (unlike RateLimit which DROPS). Critical for payment processing where losing events is unacceptable.
• Adds [Idempotency] attribute: Prevents duplicate processing using a CEL expression key. Ensures only one execution per unique key.
• Adds [Timeout] attribute: Cancels hanging functions to prevent queue buildup. Supports both Start (queue timeout) and Finish (execution timeout).

These features enable safe migration of Node.js Inngest handlers to .NET for payment processing that cannot tolerate duplicate charges or lost events.

Example Usage

[InngestFunction("payment-processor")]
[EventTrigger("payment/received")]
[Throttle(20, "1m", Key = "event.data.customerId")]  // Queue, don't drop
[Concurrency(1, Key = "event.data.paymentId")]       // Serialize per payment
[Idempotency("event.data.paymentId")]                // One execution per payment
[Timeout(Finish = "30s")]                            // Cancel if hanging
public class PaymentProcessorFunction : IInngestFunction { }

Closes #7

Test plan

• All 124 existing tests pass
• Solution builds with no errors
• Added example functions demonstrating the new attributes
• Manual test with Inngest dev server to verify registration payload

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