supabase-async: ThreadPoolExecutor에서 httpx AsyncClient로 리팩토링

This refactoring exposes a critical trap in Python async programming: wrapping synchronous calls with ThreadPoolExecutor creates false async that fails to deliver on concurrency. By migrating to httpx AsyncClient's true async I/O, the library breaks through concurrency limits (3→10 connections) while gaining 3x throughput and 28% memory savings. This pattern is essential for high-concurrency services like API gateways and web crawlers where blocking calls can become a critical bottleneck.