patchbomb: add experimental config of a "pullurl" and export it This config allows to specify a public location where your changeset can be found. It then include a dedicated patch header show a command to be used to retrieve the change. See the test for example. This is flagged as experimental because this feature is not safe until we have more logic to test that: - changeset actually exists on destination - changeset is draft on destination. As all this is experimental, bike shedding can happily happens before we remove the experimental flag.