Implement a Podfile workspace generator

When developers add CocoaPods, they need to know the transitive dependencies, and make sure those can all be built with Bazel.

Additionally, it's a lot of work for people to maintain the WORKSPACE, and know how to use this tool. We should support a WORKSPACE generator where the user just gives it a `Podfile` and then it handles the rest.

Consider minimal ways to support a `Podfile`s that will still allow a user to customize `new_pod_repository`.