Use react-native-codegen@0.0.6 in new app template

Summary:
Use pre-built react-native-codegen library from npm in the iOS app template.
Built react-native-codegen from source when used with RNTester.
Published react-native-codegen@0.0.6.

Changelog:
[iOS][Added] - Use react-native-codegen in iOS app template
[Internal] - Bump react-native-codegen: 0.0.6

Reviewed By: fkgozali

Differential Revision: D25128036

fbshipit-source-id: f294c23b9b911aae6f404edc01b62426fb578477