Continue trace from ENV vars

[antonpirker]

It should be able to continue a trace with trace information that was given to the python process over environment variables.

See this RFC for the spec: https://github.com/getsentry/rfcs/blob/main/text/0071-continue-trace-over-process-boundaries.md

[github-actions]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀