engine: resources: Add a creates field for exec

This adds a standard gate that prevents execution if a file exists. Of
note, this also adds a watch on it, so we can have a proper watched exec
resource without a watch cmd.

examples/lang/exec-creates.mcl

@@ -0,0 +1,6 @@
+exec "exec0" {
+	cmd => "echo hello world > /tmp/whatever",
+	shell => "/bin/bash",
+
+	creates => "/tmp/whatever", # a watch event is taken on this file path!
+}