1.10.0

feat: add aiohttp transport support via optional extra

The SDK now supports aiohttp as an alternative async HTTP transport backend.
When the `aiohttp` optional extra is installed (`pip install vapi_server_sdk[aiohttp]`),
the `AsyncVapi` client will automatically use `httpx-aiohttp` as its underlying
transport. Users who prefer the standard httpx client are unaffected — behaviour
is unchanged unless the extra is explicitly installed.

Key changes:
- Add new `aiohttp` optional extra (`aiohttp>=3.10,<4` + `httpx-aiohttp==0.1.8`)
- Introduce `DefaultAioHttpClient` and `DefaultAsyncHttpxClient` convenience classes (exported from top-level `vapi` package)
- Add `_make_default_async_client()` helper that auto-detects and prefers `httpx-aiohttp` when available
- Add pytest marker and conftest fixture to skip aiohttp tests when the extra is not installed

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