Introduce strategy for runner behavior

The runner model is only a class responsible for storing information
now. Based on the configuration it picks a strategy for the runner
management. The Poseidon strategy is already implemented and tested.
The Docker strategy will follow.

lib/runner/strategy.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class Runner::Strategy
+  def initialize(runner_id, environment)
+    @runner_id = runner_id
+    @execution_environment = environment
+  end
+
+  def self.request_from_management
+    raise NotImplementedError
+  end
+
+  def destroy_at_management
+    raise NotImplementedError
+  end
+
+  def copy_files(_files)
+    raise NotImplementedError
+  end
+
+  def attach_to_execution(_command)
+    raise NotImplementedError
+  end
+end