feat(core): EmbeddedFlow task

[loicmathieu]

Adds an EmbeddedFlow that allow to embed subflow tasks into a parent tasks.

After discussion with @Ben8t, the task has been called EmbeddedFlow and not TaskGroup as it better describes what it does (embedding a subflow inside a parent flow).

Fixes #6518

Limitation of the current prototypes:

• Users need to set the subflow tenantId as the subflow is sometimes fetched by methods that don't have the execution context. If it's not set, an exception will be thrown.
• Task ID from the subflow are not remapped so the task ID must be unique across the parent flow and all its subflow, transitively. A validation error will occur if it's not the case.