POC implementation for task_group dynamic dependencies - part 2 - make_edge

[kboyarinov]

Description

Second part of POC implementation for task_group dynamic dependencies.
Design described in the RFC #1664
First part is implemented in the PR #1682

This PR extends the task_group API with the new method make_edge that allows to specify predecessor-successor dependency between two tasks.
task_dynamic_state class was extended to contain current continuation vertex and the successors list.

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