Add iOS privacy info.

[yukihiro-numata]

Add PrivacyInfo to podspec of app_tracking_transparency.

By Apple's guidelines, you must add an empty file even if you are not collecting anything. ref: https://developer.apple.com/documentation/bundleresources/privacy_manifest_files

I think app_tracking_transparency doesn't collect anything, so I created an empty file.

[bazmichal]

@yukihiro-numata please add @deniza to reviewers

[deniza]

Thanks for the contribution. Our library acts a s a wrapper around AppTrackingTransparency framework and can also retrieve advertising identifier of the device by calling AppTrackingTransparency.getAdvertisingIdentifier() function. So we are technically collecting a device identifier (IDFA) for 3rd party advertising purposes.

[bazmichal]

Thank you @deniza <3