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/github.com/googleapis/gnostic/OpenAPIv2/README.md

@@ -0,0 +1,16 @@
+# OpenAPI v2 Protocol Buffer Models
+
+This directory contains a Protocol Buffer-language model
+and related code for supporting OpenAPI v2.
+
+Gnostic applications and plugins can use OpenAPIv2.proto
+to generate Protocol Buffer support code for their preferred languages.
+
+OpenAPIv2.go is used by Gnostic to read JSON and YAML OpenAPI 
+descriptions into the Protocol Buffer-based datastructures 
+generated from OpenAPIv2.proto.
+
+OpenAPIv2.proto and OpenAPIv2.go are generated by the Gnostic 
+compiler generator, and OpenAPIv2.pb.go is generated by 
+protoc, the Protocol Buffer compiler, and protoc-gen-go, the
+Protocol Buffer Go code generation plugin.