Publish copybara to maven central

Hi folks!

So we are gearing up to use this internally in a more than "Jeffry runs it on his laptop occasionally" sort of manner. We were curious if this project can get published to maven central so that we can fetch it (and new versions in the future) for use in stuff like Jenkins, etc...

Thanks again for all the great work, we've gotten a good amount of utility out of this and it's rad that you contributed this out to the open source world!