Support custom timeout.

[russbiggs]

Description

Currently the httpx Client is instantiated like this: self.client = httpx.Client() and self.client = httpx.AsyncClient() which sets the timeout to the default 5.0 seconds per https://www.python-httpx.org/advanced/timeouts/. There is additional configurability to set connect , read, write and pool timeout values using httpx.Timeout(10.0, connect=60.0) which may be helpful.