Feat/add role user access controls

[Infinite-Null]

Closes: #736

Description

This PR introduces Feature-Level Access Controls exclusively for Editor Experiments, allowing site administrators to restrict access to individual AI-powered editor features based on specific WordPress roles or individual users.

Testing Instructions

1. Navigate to AI Settings and enable Enable AI.
2. Open the Developer Tools menu (three dots) and select Access controls.
3. Under the "Editor Experiments" section, verify that the Roles and Users configuration panel appears beneath the feature toggles.
4. Check a few roles (e.g., Editor, Author) for a specific feature and click Save. Verify the save is successful.
5. In the Users field, search for a specific user and select them from the dropdown. Click Save.
6. Refresh the page and verify that your selected roles and users persisted and display their correct names.
7. Turn off "Access controls" from the Developer Tools menu, and verify that all selections are cleared.
8. Verify Enforcements: Log in to the site as a user who does not possess the selected role and is not the selected user. Open the block editor and verify that the restricted experiment is not accessible or visible to them.
9. Log in as the specifically selected user or a user with the granted role, and verify the feature is fully accessible.
10. Turn off "Access controls" from the Developer Tools menu, and verify that all selections are cleared and the feature becomes globally available again.

Screencast

User: Ankit Shah | Role: Editor

Screen.Recording.2026-06-22.at.1.35.17.PM-compressed.mp4

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Co-authored-by: Infinite-Null <ankitkumarshah@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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