[Feature Request] Add `temporalio.workflow.unsafe.in_workflow()` call

[cretz]

Describe the solution you'd like

Similar to https://www.javadoc.io/static/io.temporal/temporal-sdk/1.22.2/io/temporal/workflow/unsafe/WorkflowUnsafe.html#isWorkflowThread() or https://typescript.temporal.io/api/namespaces/workflow#inworkflowcontext, but discourage its use of course.