upload_symbols_to_crashlytics to support upload by a directory

[BoHuCB]

upload_symbols_to_crashlytics to support upload by a directory. The FirebaseCrashlytics/upload-symbols command supports command like this, "./Pods/FirebaseCrashlytics/upload-symbols -gsp ./my-app-Info.plist -p ios "./myapp/dSYMs"". It allows to upload dSYMs by directory and this command will upload all .dSYM files under the directory to firebase. However, upload_symbols_to_crashlytics only supports one file or we have to put all the paths of all the files in an array. Uploading by directory could be much easier.