As a user, I can assign a user to a manual test step in the execution screen

As a user, I can assign a user to a manual test step in the execution screen.

Some scenarios may involve different actors/users, namely in E2E tests.

The test run assignee should be able to define an assignee per step.

The system should have the possibility to notify the assignee when he is the one who has to execute the step (when the user is assigned to the step and whenever the next to be executed is assigned ot him).