feat(cros_generate_update_payload): Add features from core_upload_update
feat(cros_generate_update_payload): Add features from core_upload_update

Add the metadata generation and signature verification steps from
core_upload_update to keep similar operations together.

mgireesh05/dev-util,coreos/dev-util,mgireesh05/dev-util,coreos/dev-util,mgireesh05/dev-util,coreos/dev-util