Plugins Backend: Static validation for configuration before session start

[AmmarAbouZor]

Plugins API should provide a function to validate all the configuration and provide a detailed validation result with the ids of the invalid plugins alongside with the validation error.

This function will be called upon starting the session.

Note: Initially we can load the plugins on each call, but we need to consider to cache the loaded plugin instance for next steps.

[DmitryAstafyev]

The main idea of having this function give user good "on fly" feedback about settings, but not prevent a running of session