feat(statsig)!: Migrate to Java Core

[liran2000]

Migrated to Java Core according to Migration guide.

solves #1627.

@lindner and others you are welcome to review

[gemini-code-assist[bot]]

Summary of Changes

Hello @liran2000, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on a significant upgrade of the Statsig Java SDK within the OpenFeature provider, transitioning from the legacy serversdk to the modern javacore library. This migration involves comprehensive updates to dependencies, adaptation to new API patterns for SDK initialization and feature evaluation, and refinement of the data structures used to represent Statsig configurations. The changes ensure the provider remains compatible with the latest Statsig Java offerings and enhances its interaction with the underlying SDK.

Highlights

• Dependency Migration: The Statsig Java SDK dependency has been migrated from com.statsig:serversdk to com.statsig:javacore, updating the project to use the latest core library.
• API Adaptation: The integration with Statsig has been updated to align with the new javacore API, including changes to Statsig initialization (now instance-based instead of static calls) and how feature gates, dynamic configs, and layers are fetched and evaluated.
• StatsigUser Builder Pattern: The creation of StatsigUser objects now utilizes a builder pattern, reflecting changes in the javacore library's design.
• Object Structure Simplification: The representation of DynamicConfig and Layer objects within the OpenFeature context has been simplified, removing fields like groupName and secondaryExposures that are handled differently or no longer directly exposed by the new SDK.
• Test Suite Updates: Existing test cases have been modified to accommodate the new API and object structures, including adjustments to mocking behavior and assertion logic.

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