Added support for executing a sub-workflow

[kfrajtak]

Describe the change
This PR adds a support to run a sub-workflow from the main workflow.

Describe your implementation or design
New step builder was added to support sub-workflow. The step starts a new workflow when executed with given workflow id (name). The workflow gets an reference to main workflow, the main workflow then waits for a workflow completed event triggered from the sub-workflow and continues.

If sub-event expects an event then the event should be published with parent workflow id since the sub-workflow id is not known.

An error in the sub-workflow terminates it and consequently terminates the parent workflow as well.

Tests
Yes

Breaking change

• EndWorflow will raise the WorkflowCompleted event (which will probably solve If using EndWorkflow() no event for WorkflowCompleted LifeCycleEvent #1353)
• full exception detail is stored instead of just message in ExecutionError table