hack: generate vtproto files for buildx

Integrates vtproto into buildx. The generated files dockerfile has been
modified to copy the buildkit equivalent file to ensure files are laid
out in the appropriate way for imports.

An import has also been included to change the grpc codec to the version
in buildkit that supports vtproto. This will allow buildx to utilize the
speed and memory improvements from that.

Also updates the gc control options for prune.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

vendor/github.com/planetscale/vtprotobuf/features/grpc/grpc_plugin.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2021 PlanetScale Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package grpc
+
+import (
+	"github.com/planetscale/vtprotobuf/generator"
+	"google.golang.org/protobuf/compiler/protogen"
+)
+
+const version = "1.1.0-vtproto"
+
+var requireUnimplementedAlways = true
+var requireUnimplemented = &requireUnimplementedAlways
+
+func init() {
+	generator.RegisterFeature("grpc", func(gen *generator.GeneratedFile) generator.FeatureGenerator {
+		return &grpc{gen}
+	})
+}
+
+type grpc struct {
+	*generator.GeneratedFile
+}
+
+func (g *grpc) GenerateFile(file *protogen.File) bool {
+	if len(file.Services) == 0 {
+		return false
+	}
+
+	generateFileContent(nil, file, g.GeneratedFile)
+	return true
+}