[native] add CXX header files to Xcode project

Summary:
There doesn't seem to be a straightforward way to add generated header files to our Xcode project without checking them in. This change lets users easily call rust functions declared in the lib.rs.h header file.

Depends on D4953

Test Plan: Built the mobile app in Xcode

Reviewers: atul, jon, tomek

Reviewed By: atul, jon, tomek

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5084