Scoped validators

[jakubfijalkowski]

As a result of a discussion on topic presented by @konradbartecki-45e6 here, it will be a good idea to move away from singleton validators or at least allow one to choose (e.g. per app) if they want to register validators as singletons or as scoped services.

Scoped validators will allow us to remove most of the extensions on FluentValidation, as we would no longer need to inject DI context or anything like that. We can also use this opportunity to upgrade FluentValidation to newer version.