feat: add setTrackAssignmentEvent

[zhukaihan]

Summary

Add setTrackAssignmentEvent.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: no

[Copilot]

Pull Request Overview

This PR adds a setTrackAssignmentEvent method to the Amplitude Experiment JavaScript Client SDK that allows users to control whether assignment events are tracked when fetching experiment variants.

• Introduces a new setTrackAssignmentEvent method that persists tracking preferences to storage
• Modifies the fetch logic to include tracking options from stored preferences
• Adds comprehensive test coverage for the new functionality

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 3 comments.

File	Description
packages/experiment-browser/src/experimentClient.ts	Implements the setTrackAssignmentEvent method and integrates tracking options into the fetch flow
packages/experiment-browser/src/storage/cache.ts	Adds SingleValueStoreCache class and getVariantsOptionsStorage utility for persisting fetch options
packages/experiment-browser/src/storage/cookie-storage.ts	Adds a new CookieStorage implementation for the Storage interface
packages/experiment-browser/test/client.test.ts	Comprehensive test suite covering all aspects of the new setTrackAssignmentEvent functionality

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📝 Documentation updates detected!

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