firebase-ios-sdk

 Add option to generate Carthage build.  (#3517)

* Add option to generate Carthage build.

Still a few things to do here, but the structure is there.

* Revert unused changes.

* Further Carthage work.

* Hashing the contents of the directory for naming.

* Update README for Carthage.

* Change hash calculation to SHA256 for Carthage naming.

* Undo accidental change

* Generate Firebase pod for Carthage

* Parsing JSON files.

* Include RC number, new output naming.

* Further iterations on getting Carthage working.

* Fix long line.

* Undo version removal.

* Proper parsing of JSON manifest.

* Fix JSON output, properly copy to outputDir.

* Replace CoreDiagnostics framework with newly built one.

* Clear output directory if it doesn't exist.

* Fix the JSON formatting.

* Added newline before closing JSON.