Make system context optional

[frankong]

Hi team, we currently use Posthog for our frontend and are exploring using it for Python backend error tracking and llm observability.

One issue we are having is that backend will capture OS and OS platform in system context which will always be Linux:

posthog-python/posthog/client.py

Line 328 in 18729e3

properties = {**(properties or {}), **system_context()}

This causes issues in our dashboards and metrics. Would it be possible to make system context optional?