Adds a --private-key option to the build command, which specifies an ECDSA
private key. When this is provided along with a manifest, the manifest is
prepended to the .flx package and signed with the private key. The manifest
also includes a SHA-256 hash of the zipped content portion of the .flx package.

This is used by the Flutter updater package, to verify that updates are
from the right publisher.