feat: Support for polymorphic task.run, task.run_async.

We can now use this syntax:

result = task[DesiredType].run(msg, ...)  # similar for run_async

where msg can be of any type and DesiredType is the desired type of the task result.

See:

• tests: https://github.com/langroid/langroid/blob/main/tests/main/test_task_run_polymorphic.py
• example: https://github.com/langroid/langroid/blob/main/examples/basic/multi-agent-search-critic-no-orch/main.py