Add missed mockInfoGenerator for system static objects

[CaelmBleidd]

Add missed mock information for static values of system classes

Description

Removed a way to create a static object without explicitly providing mockInfoGenerator, added a required one for creating of system static objects

Fixes #2015

How to test

Automated tests

org.utbot.examples.mock.MockStaticMethodExampleTest#testMockStaticMethodFromAlwaysMockClass

Manual tests

Add a system class to the mockAlways list, and run the generation of a code containing static method invocation. At least one branch should be found.

Self-check list

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.