Make build order constants public

### Please fill in the following fields:
Unity editor version: 2019.2.18
External Dependency Manager version: 1.2.153
Source you installed EDM (from .unitypackage or Unity Package Manager): UPM
Features in External Dependency Manager in use (Android Resolver, iOS Resolver, VersionHandler, etc.): iOS Resolver
Plugins SDK in use (Firebase, Admob, Facebook, etc.):
Platform you are using the Unity editor on (Mac, Windows, or Linux): macOS
Platform you are targeting (iOS, Android, and/or desktop): iOS
Scripting Runtime (Mono, and/or IL2CPP):

### Please describe the issue here:
We have some postprocess steps that modified cocoapods' input and checked the output, but these were broken with the last [build order change](https://github.com/googlesamples/unity-jar-resolver/commit/91e8010c475e784d7d1758f59961586229f0f2ec). Could you make these constants public so we can reference them directly?
For instance you could change the sample from `[PostProcessBuildAttribute(45)]` to something like `[PostProcessBuildAttribute(IOSResolver.BUILD_ORDER_GEN_PODFILE + 1)]` and you would have freedom to change the values in the future lowering the chances of breaking other 3rd party postprocessors :)

### Please answer the following, if applicable:
What's the issue repro rate? (eg 100%, 1/5 etc)