Use CocoaPods CDN to speed up pod install

### Please fill in the following fields:
Unity editor version: 2019.3.0f6
Play Services Resolver version: 1.2.135.0
Features in Play Services Resolver in use (Android Resolver, iOS Resolver, VersionHandler, etc.): iOS Resolver
Plugins SDK in use (Firebase, Admob, Facebook, etc.): Firebase
Platform you are using the Unity editor on (Mac, Windows, or Linux): Mac
Platform you are targeting (iOS, Android, and/or desktop): iOS
Scripting Runtime (Mono, and/or IL2CPP): IL2CPP

### Please describe the issue here:
On some CI services (in my case TravisCI) sometimes the `pod install` step can slow to a crawl. This is due to GitHub throttling traffic to the CocoaPods/Specs.git source repo.

CocoaPods have since released support to use a CDN ([in CocoaPods 1.7.2+](https://blog.cocoapods.org/CocoaPods-1.7.2/)). To opt-in, the `source` at the top of the `Podfile` should be changed over to `https://cdn.cocoapods.org/`.

The straightforward change to `unity-jar-resolver` would be to update this hardcoded string in IOSResolver to point to the CDN:

https://github.com/googlesamples/unity-jar-resolver/blob/master/source/IOSResolver/src/IOSResolver.cs#L80

However this is a breaking change since it drops support for CocoaPods < 1.7.2. Perhaps the better solution would be to allow configuring this URL in our build scripts before running the build.

Any thoughts?