Distributed tracing by CLI scripts

**Is your feature request related to a problem? Please describe.**
We would like to add distributed tracing to CLI scripts. We have a scheduler process that starts separate PHP processes. This scheduler reports back runtime status among other things.
It would be great if we can pass the traceId + transactionId of the scheduler script, to the new PHP processes. Where it would be set as traceId + parentId.

**Describe the solution you'd like**
The extension not only looks for request headers, but also for an ENV var.
This way we can pass the tracecontext of the scheduler script to the new spawned processes.