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/google.golang.org/appengine/internal/modules/modules_service.proto

@@ -0,0 +1,80 @@
+syntax = "proto2";
+option go_package = "modules";
+
+package appengine;
+
+message ModulesServiceError {
+  enum ErrorCode {
+    OK  = 0;
+    INVALID_MODULE = 1;
+    INVALID_VERSION = 2;
+    INVALID_INSTANCES = 3;
+    TRANSIENT_ERROR = 4;
+    UNEXPECTED_STATE = 5;
+  }
+}
+
+message GetModulesRequest {
+}
+
+message GetModulesResponse {
+  repeated string module = 1;
+}
+
+message GetVersionsRequest {
+  optional string module = 1;
+}
+
+message GetVersionsResponse {
+  repeated string version = 1;
+}
+
+message GetDefaultVersionRequest {
+  optional string module = 1;
+}
+
+message GetDefaultVersionResponse {
+  required string version = 1;
+}
+
+message GetNumInstancesRequest {
+  optional string module = 1;
+  optional string version = 2;
+}
+
+message GetNumInstancesResponse {
+  required int64 instances = 1;
+}
+
+message SetNumInstancesRequest {
+  optional string module = 1;
+  optional string version = 2;
+  required int64 instances = 3;
+}
+
+message SetNumInstancesResponse {}
+
+message StartModuleRequest {
+  required string module = 1;
+  required string version = 2;
+}
+
+message StartModuleResponse {}
+
+message StopModuleRequest {
+  optional string module = 1;
+  optional string version = 2;
+}
+
+message StopModuleResponse {}
+
+message GetHostnameRequest {
+  optional string module = 1;
+  optional string version = 2;
+  optional string instance = 3;
+}
+
+message GetHostnameResponse {
+  required string hostname = 1;
+}
+