new driver: kubernetes

Tested with `kind` and GKE.

Note: "nodes" shown in `docker buildx ls` are unrelated to Kubernetes "nodes".
Probably buildx should come up with an alternative term.

Usage:

  $ kind create cluster
  $ export KUBECONFIG="$(kind get kubeconfig-path --name="kind")"

  $ docker buildx create --driver kubernetes --driver-opt replicas=3 --use
  $ docker buildx build -t foo --load .

`--load` loads the image into the local Docker.

Driver opts:

  - `image=IMAGE` - Sets the container image to be used for running buildkit.
  - `namespace=NS` - Sets the Kubernetes namespace. Defaults to the current namespace.
  - `replicas=N` - Sets the number of `Pod` replicas. Defaults to 1.
  - `rootless=(true|false)` - Run the container as a non-root user without `securityContext.privileged`. Defaults to false.
  - `loadbalance=(sticky|random)` - Load-balancing strategy. If set to "sticky", the pod is chosen using the hash of the context path. Defaults to "sticky"

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

vendor/cloud.google.com/go/CONTRIBUTORS

@@ -0,0 +1,40 @@
+# People who have agreed to one of the CLAs and can contribute patches.
+# The AUTHORS file lists the copyright holders; this file
+# lists people.  For example, Google employees are listed here
+# but not in AUTHORS, because Google holds the copyright.
+#
+# https://developers.google.com/open-source/cla/individual
+# https://developers.google.com/open-source/cla/corporate
+#
+# Names should be added to this file as:
+#     Name <email address>
+
+# Keep the list alphabetically sorted.
+
+Alexis Hunt <lexer@google.com>
+Andreas Litt <andreas.litt@gmail.com>
+Andrew Gerrand <adg@golang.org>
+Brad Fitzpatrick <bradfitz@golang.org>
+Burcu Dogan <jbd@google.com>
+Dave Day <djd@golang.org>
+David Sansome <me@davidsansome.com>
+David Symonds <dsymonds@golang.org>
+Filippo Valsorda <hi@filippo.io>
+Glenn Lewis <gmlewis@google.com>
+Ingo Oeser <nightlyone@googlemail.com>
+James Hall <james.hall@shopify.com>
+Johan Euphrosine <proppy@google.com>
+Jonathan Amsterdam <jba@google.com>
+Kunpei Sakai <namusyaka@gmail.com>
+Luna Duclos <luna.duclos@palmstonegames.com>
+Magnus Hiie <magnus.hiie@gmail.com>
+Mario Castro <mariocaster@gmail.com>
+Michael McGreevy <mcgreevy@golang.org>
+Omar Jarjur <ojarjur@google.com>
+Paweł Knap <pawelknap88@gmail.com>
+Péter Szilágyi <peterke@gmail.com>
+Sarah Adams <shadams@google.com>
+Thanatat Tamtan <acoshift@gmail.com>
+Toby Burress <kurin@google.com>
+Tuo Shan <shantuo@google.com>
+Tyler Treat <ttreat31@gmail.com>