Implement the support for adhoc commands

We have two basic example commands. But it’s not entirely finished because
there are some error checks that we don’t do.

ref #2521

src/xmpp/adhoc_command.cpp

@@ -0,0 +1,82 @@
+#include <xmpp/adhoc_command.hpp>
+
+using namespace std::string_literals;
+
+AdhocCommand::AdhocCommand(std::vector<AdhocStep>&& callbacks, const std::string& name, const bool admin_only):
+  name(name),
+  callbacks(std::move(callbacks)),
+  admin_only(admin_only)
+{
+}
+
+AdhocCommand::~AdhocCommand()
+{
+}
+
+void PingStep1(AdhocSession&, XmlNode& command_node)
+{
+  XmlNode note("note");
+  note["type"] = "info";
+  note.set_inner("Pong");
+  note.close();
+  command_node.add_child(std::move(note));
+}
+
+void HelloStep1(AdhocSession&, XmlNode& command_node)
+{
+  XmlNode x("jabber:x:data:x");
+  x["type"] = "form";
+  XmlNode title("title");
+  title.set_inner("Configure your name.");
+  title.close();
+  x.add_child(std::move(title));
+  XmlNode instructions("instructions");
+  instructions.set_inner("Please provide your name.");
+  instructions.close();
+  x.add_child(std::move(instructions));
+  XmlNode name_field("field");
+  name_field["var"] = "name";
+  name_field["type"] = "text-single";
+  name_field["label"] = "Your name";
+  XmlNode required("required");
+  required.close();
+  name_field.add_child(std::move(required));
+  name_field.close();
+  x.add_child(std::move(name_field));
+  x.close();
+  command_node.add_child(std::move(x));
+}
+
+void HelloStep2(AdhocSession& session, XmlNode& command_node)
+{
+  // Find out if the name was provided in the form.
+  XmlNode* x = command_node.get_child("x", "jabber:x:data");
+  if (x)
+    {
+      XmlNode* name_field = nullptr;
+      for (XmlNode* field: x->get_children("field", "jabber:x:data"))
+        if (field->get_tag("var") == "name")
+          {
+            name_field = field;
+            break;
+          }
+      if (name_field)
+        {
+          XmlNode* value = name_field->get_child("value", "jabber:x:data");
+          if (value)
+            {
+              XmlNode note("note");
+              note["type"] = "info";
+              note.set_inner("Hello "s + value->get_inner() + "!"s);
+              note.close();
+              command_node.delete_all_children();
+              command_node.add_child(std::move(note));
+              return;
+            }
+        }
+    }
+  // TODO insert an error telling the name value is missing.  Also it's
+  // useless to terminate it, since this step is the last of the command
+  // anyway. But this is for the example.
+  session.terminate();
+}