[FSSDK-11552] Add holdout support and refactor decision logic

[Mat001]

Per jira ticket:

• Add a new method named getVariationFromHoldout and implement holdout evaluation logic.
• Update the getVariationForFeatureExperiment method to incorporate holdout evaluation logic.

Changes:

1. pkg/entities/experiment.go - Added Holdout entity
2. pkg/config/interface.go - Added GetHoldoutsForFlag interface method
3. pkg/config/datafileprojectconfig/config.go - Implemented stub
4. pkg/decision/entities.go - Added Holdout source constant
5. pkg/decision/holdout_service.go - NEW holdout service implementation
6. pkg/decision/composite_feature_service.go - Integrated holdout service
7. pkg/decision/composite_feature_service_test.go - Updated test expectations
8. pkg/decision/evaluator/audience_evaluator_test.go - Added mock method
9. pkg/cmab/service_test.go - Added mock method

Jira ticket:
https://optimizely-ext.atlassian.net/browse/FSSDK-11552

[prisma-cloud-devsecops]

Prisma Cloud has found errors in this PR ⬇️

[prisma-cloud-devsecops]

  Incorrect access of indexable resource
    File: holdout_service.go | Checkov ID: CKV3_SAST_261

Description

CWE: CWE-118: Incorrect Access of Indexable Resource ('Range Error')
OWASP: A05:2021-Security Misconfiguration