Implement support for sub-workflow execution

[treblereel]

This feature request proposes adding support for sub-workflow execution through the new RunTask construct

Expected behavior:

• The parent workflow pauses until the sub-workflow completes.
• The sub-workflow result becomes the output of the RunTask
• Errors and context updates are propagated consistently with other task types.

Motivation

• Enable workflow composition and reuse of existing definitions.
• Simplify large workflows by decomposing them into smaller, testable units.
• Improve maintainability and alignment with the Serverless Workflow DSL specification.