support declaring RoslynCodeTaskFactory tasks multithreadable

this is a common extension mechanism used by our customers and we should support them in making inline tasks multithreadable to avoid sidecar processes

Start by tracing how RoslynCodeTaskFactory tasks and inline tasks are declared, then inspect the current sidecar-process behavior. Determine where multithreadability can be declared and how support should be verified; done means customers can declare inline tasks multithreadable without requiring sidecar processes.

Written by the indexing model from the issue text.