storage manager: add rclone backend.

app/logical/storage_manager/rclone.rb

@@ -0,0 +1,34 @@
+class StorageManager::Rclone < StorageManager
+  class Error < StandardError; end
+  attr_reader :remote, :bucket, :rclone_options
+
+  def initialize(remote:, bucket:, rclone_options: {}, **options)
+    @remote = remote
+    @bucket = bucket
+    @rclone_options = rclone_options
+    super(**options)
+  end
+
+  def store(file, path)
+    rclone "copyto", file.path, key(path)
+  end
+
+  def delete(path)
+    rclone "delete", key(path)
+  end
+
+  def open(path)
+    file = Tempfile.new(binmode: true)
+    rclone "copyto", key(path), file.path
+    file
+  end
+
+  def rclone(*args)
+    success = system("rclone", *rclone_options, *args)
+    raise Error, "rclone #{$?}" if !success
+  end
+
+  def key(path)
+    ":#{remote}:#{bucket}#{path}"
+  end
+end