Swarming protos

Source:

• Repo: https://chromium.googlesource.com/infra/luci/luci-py
• Path: appengine/swarming/proto
• Revision: cf408cb4d1ce3b4736ef39caa1199defac1bae1b

Updating

1. Run ./update.sh
2. Update README.md with git commit hash printed.
3. NOTE: Due to a discrepancy between luci-go and luci-py, you also need to revert the luci.file_metadata file option from config/*.proto.
4. Run go generate ./...