Add registry mirror support in Jib build plugins

[chanseokoh]

Towards #1396.

Supports only public mirrors such as mirror.gcr.io. (If a mirror requires credentials, it won't work.)

I went with the simple approach to try the mirrors when pulling a base image manifest in PullBaseImageStep. The class sets up a RegistryClient and returns it, so we just try pulling from mirrors and the actual registry until success. (So, this assumes that if a mirror returns a manifest with 200, all subsequent requests to pull blobs will succeed, which is not an unreasonable assumption.)

Tested with the following config:

{
  "disableUpdateCheck":false,
  "registryMirrors": [
    {
      "registry": "registry-1.docker.io",
      "mirrors": ["localhost:5000", "mirror.gcr.io"]
    }
  ]
}

In my test, the first mirror localhost:5000 always fails (connection error), but it doesn't fail the entire build. It tries mirror.gcr.io next. Then, if mirror.gcr.io has cached the base image, Jib nevers contact Docker Hub. If mirror.gcr.io returns 404, it correctly falls back to Docker Hub.

[chanseokoh]

Added tests and ready for review. But #3010 needs to go first.

Will update CHANGELOG in the meantime.