Add date column to the built-in search engine

Adds a date column to the built-in search engine to show when a torrent was published/uploaded on the engine site.
When a plugin wants to show a date, it can now add a `pub_date` entry to its result dict. The value format is a unix timestamp (an integer representing seconds since epoch).
Plugins with no date support will keep working.

PR #20703.

src/base/search/searchhandler.h

@@ -30,6 +30,7 @@
 #pragma once
 
 #include <QByteArray>
+#include <QDateTime>
 #include <QList>
 #include <QObject>
 #include <QString>
@@ -47,6 +48,7 @@ struct SearchResult
     qlonglong nbLeechers = 0;
     QString siteUrl;
     QString descrLink;
+    QDateTime pubDate;
 };
 
 class SearchPluginManager;