ConfigServer: Monitor opened config files for changes

After opening a domain configuration and putting into our configuration
cache, we now monitor the underlying file for changes and send out
notifications to monitoring clients as needed.

This patch has three FIXME's:

- We create a new Core::FileWatcher for each domain.
- We don't yet detect removed keys.
- We don't know the type of key, so we assume everything is a string.

There are a number of ways we can solve those problems but let's not
hold up this patch while we wait for solutions. :^)

Userland/Services/ConfigServer/ClientConnection.h

@@ -20,6 +20,8 @@ public:
 
     virtual void die() override;
 
+    bool is_monitoring_domain(String const& domain) const { return m_monitored_domains.contains(domain); }
+
 private:
     explicit ClientConnection(NonnullRefPtr<Core::LocalSocket>, int client_id);