add privacy manifest to pod install

Summary:
Changelog: [iOS][Added]

this creates the RN privacy manifest in the ios build step if user has not created one yet. the reasons have been added for the following APIs:

NSPrivacyAccessedAPICategoryFileTimestamp
- C617.1: We use fstat and stat in a few places in the C++ layer. We use these to read information about the JavaScript files in RN.

NSPrivacyAccessedAPICategoryUserDefaults
- CA92.1: We access NSUserDefaults in a few places.
1) To store RTL preferences
2) As part of caching server URLs for developer mode
3) A generic native module that wraps NSUserDefaults

NSPrivacyAccessedAPICategorySystemBootTime
- 35F9.1: Best guess reason from RR API pulled in by boost

Reviewed By: cipolleschi

Differential Revision: D53687232

fbshipit-source-id: 6dffb1a6013f8f29438a49752e47ed75c13f4a5c

# Conflicts:
#	packages/rn-tester/RNTesterPods.xcodeproj/project.pbxproj