Global timeout

[andybbruno]

Summary

This pull request introduces a new, more straightforward way to configure a global default timeout for all clients and requests in httpx without needing to access the internal _config object. The enhancement is achieved by exposing the DEFAULT_TIMEOUT_CONFIG in the main __init__.py file (see discussion here).

This change simplifies the process, allowing developers to set a global timeout directly and elegantly, like so:

import httpx
httpx.DEFAULT_TIMEOUT_CONFIG = httpx.Timeout(10.0)

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.