feat: add async tracking permission API

[2sem]

Goal

Provide an async App Tracking Transparency permission API while preserving the existing callback API.

Changes

• Add requestPermissionAsync(...) async -> ATTrackingManager.AuthorizationStatus
• Bridge the callback result with withCheckedContinuation
• Ensure the .restricted path invokes completion so the continuation cannot hang
• Document callback and async usage

User flow

flowchart LR
    Caller[Async caller] --> Async[requestPermissionAsync]
    Async --> Callback[Existing requestPermission callback API]
    Callback --> ATT[ATT authorization result]
    ATT --> Resume[Resume checked continuation once]
    Resume --> Caller

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Success criteria

• Existing callback callers remain source-compatible
• Async callers receive the ATT authorization status
• Every authorization path resumes the continuation exactly once
• Scope remains limited to one method family

Verification

• xcodebuild build -workspace .swiftpm/xcode/package.xcworkspace -scheme GADManager -destination generic/platform=iOS Simulator CODE_SIGNING_ALLOWED=NO passed
• 0 build errors and 0 warnings through xcsift
• Independent code review approved with no findings

Risks and rollback

• A distinct requestPermissionAsync name avoids changing overload resolution for existing no-callback calls in async contexts.
• Rollback is a direct revert of this PR; the callback API remains intact.

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