feat: add async HTTP transport foundation (Stage 0)

[LioriE]

What

Lays the foundation for async support in the Python SDK as the first stage of the async rollout plan.

future_utils.py — three transparent helpers:

• then(result_or_coro, fn) — applies fn immediately on a sync result, or returns an awaitable that applies it after the coroutine resolves
• wrap(result, as_awaitable) — wraps a sync value in a coroutine when needed
• resolve(obj) — awaits if async, returns as-is if sync

HTTPClient — async execution path alongside the existing sync path:

• Persistent httpx.AsyncClient instance created when async_mode_experimental=True
• _async_execute_with_retry mirrors the sync retry loop using asyncio.sleep
• All five public methods (get/post/put/patch/delete) accept async_mode: bool = False; passing True delegates to the async counterpart and returns a coroutine

DescopeClient.__init__ — accepts async_mode_experimental via **kwargs and forwards it to both HTTP client instances, ready for the eventual global-rollout stage

Sync callers are completely unaffected — all existing tests pass unchanged.

Related

descope/etc#5922