fix: add AB test variant support for feature flags

[aixaCode]

What does this PR change?

To properly support AB testing, we need feature flag variants to distinguish which experiment is used.

• fixed JSON serialization of FeatureFlagVariant class
• added support for defining variant name in featureFlag name, i.e test_feature_flag_variant:variantName
• updated feature flag name with _variant suffix, that are used in A/B test to support format needed for the analytics pipeline

Kernel PR More info here and how to setup in Unleash: https://github.com/decentraland/adr/blob/main/docs/ADR-73-ab-testing.md

How to test the changes?

Use this link to test changes https://play.decentraland.zone/?renderer-branch=fix/add-ab-test-variant-feature-flag?kernel-branch=fix%2Ffeature-flag-variant-parsing Test with additional params

• ENABLED_UI_BLUR_VARIANT, DISABLED_UI_BLUR_VARIANT
• ENABLED_USE_GPU_ONLY_MESH_VARIANT, DISABLED_USE_GPU_ONLY_MESH_VARIANT
  • no visible change should be seen

Our Code Review Standards

https://github.com/decentraland/unity-renderer/blob/master/docs/code-review-standards.md

[github-actions[bot]]

After the CI passes:

• This branch can be previewed at https://play.decentraland.zone/?renderer-branch=fix/add-ab-test-variant-feature-flag
• Code coverage report: https://renderer-artifacts.decentraland.org/branch-coverage/fix/add-ab-test-variant-feature-flag
• Benchmark report: https://renderer-artifacts.decentraland.org/branch-benchmark/fix/add-ab-test-variant-feature-flag/index.html