VespaAsync _open_aiohttp_session method doesn't need to be async

[PrathamSoni]

Would remove some marginal asyncio overhead if the method was made synchronous. It can still be called in aenter context with the same performance as the code is blocking anyways.

Would be helpeful in terms of overall design patterns where similar long-running clients can all be started/managed together as such "start" methods are not async in nature (just creating some httpx/aiohttp client as is done here)