Implement DBOS.run_step for in-line steps

[qianl15]

Currently, DBOS Python has to use the @DBOS.step decorator if we want to execute a function as a step.
In DBOS TypeScript, we have a powerful DBOS.runStep function, allowing users to run a function as a step (even an inline function) in a workflow without decoration.

Would be a nice feature to add to Python.