Codesigning Script

This script will--given a particular Flutter Engine revision--fetch the
necessary cache artifacts and codesign & notarize them. Pre-requisites are:

1. You must have gsutil installed and authenticated to write to the
   flutter-infra bucket.
2. The following env variables must be set: CODESIGN_USERNAME,
   CODESIGN_CERT_NAME, APP_SPECIFIC_PASSWORD. There is an internal doc that
   has more information on these.
3. Xcode must be installed, and a Developer ID certificate must be present
   in the keychain (the name of this cert should be CODESIGN_CERT_NAME)

Usage is as follows:

./codesign.py <engine_revision_hash>