Add privacy manifest

[jaredmixpanel]

This PR adds a privacy manifest to declare the use of the required API for UserDefaults with the following reason:

1C8F.1
Declare this reason to access user defaults to read and write information that is only accessible to the apps, app extensions, and App Clips that are members of the same App Group as the app itself.

This reason does not permit reading information that was written by apps, app extensions, or App Clips outside the same App Group or by the system. This reason also does not permit writing information that can be accessed by apps, app extensions, or App Clips outside the same App Group.

Addresses: #620

[zihejia]

LGTM