Make it easier to reuse connections

[andreer]

Currently in our fasthtml demo app, each call to Vespa uses the pattern

    async def get_something_from_vespa(self, doc_id: str) -> str:
        async with self.app.asyncio(connections=1) as session:
            ...

This ends up opening a new connection, doing TLS handshake for every request.

Quoting the doc, the session "should be used as a session manager" but this means we can't really store and reuse it.

Perhaps we can pass a httpx client which can be (re)used for the session?