firebase-ios-sdk

Introduce Zip File Builder (in Swift) (#2407)

* Introduce tooling to build the Zip file.

* Cleanup to prepare for review.

This moves argument parsing into its own file, moves some methods
into CocoaPodUtils and consolidates some related types.

* Add modulemap and Firebase.h

* Adding CoreDiagnostics, WIP

* Refactored paths into separate struct.

* Organized methods.

* Remove unused utility import.

* Properly passed in custom spec repos.

* Added licenses, outputDir argument.

* Fix the Zip command to use the working directory.

* Fixing bundles and headers being copied.

* Fixing the resources directories.

* More iterations to come closer to Released zip.

- Fixed the caching of OSS pods
- Add custom Crash objects
- Move resources around properly.

* Missing comment.

* Fix resources for MLKit and others.

* Remove unused URL extension.

* Add priorities to README, exclude Firebase.framework

* Run style.sh

* Fully ran style.sh

* Add license to files that don't have it.

* Address feedback from first review.

* Add missing custom spec repo flag.