Populate and propagate dynamic sampling context when Python is head of trace

[vladanpaunovic]

Problem Statement

Many server side rendering applications start traces on the server, not on the client. So the head SDK is more often then not a a server side one, not a frontend SDK as we assumed.
This means that server side applications don't have way of setting the dynamic sampling context at the right time.

Solution Brainstorm

Populate and propagate dynamic sampling context if SDK is head of trace.

Related javascript PR getsentry/sentry-javascript#5193