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/moby/buildkit/solver/pb/ops.proto

@@ -341,6 +341,8 @@ message FileActionCopy {
 	repeated string exclude_patterns = 13;
 	// alwaysReplaceExistingDestPaths results in an existing dest path that differs in type from the src path being replaced rather than the default of returning an error
 	bool alwaysReplaceExistingDestPaths = 14;
+	// mode in non-octal format
+	string modeStr = 15;
 }
 
 message FileActionMkFile {