Add validation groups support

We should provide the ability to apply different validation rules depending on particular validation groups. To achieve this we should do the following:

• extend ValidatableReference#validate and ValidatableCollection#validateEach with argument Class<?>... groups - this way we'll configure each validation rules set to be used with particular validation groups
• extend ValidatableObject#validate with argument Class<?>... groups - this way we'll trigger validation according to passed validation groups

Start by locating ValidatableReference#validate, ValidatableCollection#validateEach, and ValidatableObject#validate, then trace how validation rules are selected and invoked. Define how the Class<?>... groups arguments propagate through these APIs and verify that each validation set runs only for its selected groups. The issue names no tests, so inspect existing validation tests or generated-code coverage before adding focused cases.