Plugins Frontend: Configuration validation before starting the session

[AmmarAbouZor]

This PR is blocked by #2211

After the validation function is implemented, we need to change the behavior before starting plugins sessions and call validation function to ensure configs are valid before starting the session.
The function for plugins will expect to get all plugin configuration and will return detailed validation info.

Note: The validation at this step should be called upon starting the session and won't support live validation