Add/scaffold Privacy Sandbox datastore

[tofumatt]

Feature Description

A new datastore for the Privacy Sandbox, named modules/privacy-sandbox should be created.

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Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• A new datastore for the Privacy Sandbox should be created in assets/js/modules/privacy-sandbox
• A new constant (MODULES_PRIVACY_SANDBOX) should export the module name 'modules/privacy-sandbox' in assets/js/modules/privacy-sandbox/constants.js
• The datastore should be registered when the privacySandboxModule feature flag is enabled. It should not be registered when the feature flag is disabled.
• The datastore should be scaffolded with the required registry.registerStore type code (see PageSpeed's for example), but without any file stores with specific functionality.

Implementation Brief

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Test Coverage

QA Brief

Changelog entry

[tofumatt]

I’ve unassigned @zutigrm while this is marked "Stalled", but whenever this is moved from Stalled back to IB: check with @zutigrm before taking over the issue, since @zutigrm might have started on it already 👍🏻