Add an async client on httpx with timeouts and cancellable polling #27

The generated client uses synchronous requests with no timeout, and _poll_for_completion sleeps in a loop. Inside an async server such as an arcade-mcp-server tool handler, every Turnkey call blocks the event loop for the whole request plus polling.

Follow-up:

• Generate an AsyncTurnkeyClient next to TurnkeyClient from the same codegen, on httpx.AsyncClient.
• Add explicit connect and read timeouts to both clients, with sensible defaults.
• Make polling honor cancellation and expose max_polling_retries and polling_interval_ms clearly; the current defaults (3 retries at 1 second) are too short for most activities.
• Keep the sync client; let it share the request building, stamping, and result unpacking code so the two cannot drift.

Depends on #20 and #21.