[iOS] Add Privacy Manifest to BraintreeCore module

[agedd]

Summary of changes

• Adds the PrivacyInfo.xcprivacy file to the BraintreeCore module
• Updates the Braintree.podspec and Package.swift files to include references to the Privacy Manifest file

Checklist

• ~Added a changelog entry~

Authors

List GitHub usernames for everyone who contributed to this pull request.

• @agedd