Cherry pick PR #9477: starboard: Refactor h5vcc plumbing to use a dedicated struct and extension

[cobalt-github-releaser-bot]

Refer to the original PR: #9477

This change consolidates individual experimental features into a single 'ExperimentalFeatures' struct, updates the player worker handler interface to accept this struct, and introduces a new Starboard extension for cross-platform configuration. The Android implementation has been updated to use this new pattern, and redundant internal configuration headers have been removed.

Issue: 480134029

[cobalt-github-releaser-bot]

Caution

There were merge conflicts while cherry picking! Check out cherry-pick-main-9477 and fix the conflicts before proceeding. Check the log at https://github.com/youtube/cobalt/actions/runs/24424649716 for details.

[github-actions]

🤖 Gemini Suggested Commit Message

---

starboard: Refactor experimental features plumbing

Consolidate individual experimental features into a single
ExperimentalFeatures struct and a new ExperimentalFeaturesConfigurationApi
Starboard extension. The player worker handler interface and Android
implementation are updated to use this new pattern.

This change simplifies the management and cross-platform configuration of
experimental features, moving towards a more organized and extensible
approach. It removes redundant internal configuration headers and
extensions, centralizing the configuration of experimental flags.

Bug: 480134029

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[gemini-code-assist]

Code Review

This pull request consolidates several specific Starboard extensions into a unified ExperimentalFeatures extension, simplifying the SbPlayer creation process and utilizing thread-local storage for feature management. The review feedback identifies a critical crash risk in the feature retrieval logic when overrides are absent, a type inconsistency in the extension header that prevents boolean features from being optional, and a regression in debug logging visibility that should be addressed to maintain field supportability.