Use a memoized Flipper engine in specs to simplify testing

What does this MR do?

This predates the !31610 (closed).

It makes the following changes:

• Use a memoized Flipper engine in tests instead of stubs everywhere
• Usage of a memoized Flipper engine allows us to have a by-default support for all Flipper aspects
• We also change a implementation of persisted_names to use a Flipper instead of requesting features from DB: this is likely even cheaper, as we would get data from process/redis storage first
• The usage of memoized engine can be overwritten with stub_feature_flags: false
• The memoized engine requires us to use object that are FeatureGate-inherited
• We fix all tests to ensure that they adhere to that

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content