Async version of feed_async_iterable

[kevinhu]

Despite making async calls, Vespa.feed_async_iterable is itself a synchronous method.

When using Vespa inside an asynchronous context such as a web server, this runs into the classic RuntimeError: asyncio.run() cannot be called from a running event loop error because feed_async_iterable defines a run() wrapper that's passed to asyncio.run to make the method synchronous. As a result, it's tricky to run parallel operations without organizing the tasks yourself.

A possible solution is to give VespaAsync it's own feed_async_iterable method.