config: Convert 'protected_packages' to an append option

This change is necessary because the list of protected packages started
being distributed across configuration files within a drop-in directory.
This led to a situation where a subsequent configuration file containing
`protected_packages` would overwrite any earlier settings.

This issue was previously encountered and resolved in **libdnf5**
approximately two years ago by changing `protected_packages` to
an append option. The same proven solution is implemented here to ensure
correct merging and prevent unexpected overwrites of earlier
configurations.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2400488

Author: jrohel

libdnf/conf/ConfigMain.cpp

@@ -514,11 +514,11 @@ ConfigMain::Impl::Impl(Config & owner)
     owner.optBinds().add("proxy_username", proxy_username);
     owner.optBinds().add("proxy_password", proxy_password);
     owner.optBinds().add("proxy_auth_method", proxy_auth_method);
+
     owner.optBinds().add("protected_packages", protected_packages,
         [&](Option::Priority priority, const std::string & value){
-            if (priority >= protected_packages.getPriority())
-                protected_packages.set(priority, resolveGlobs(value));
-        }, nullptr, false
+            optionTListAppend(protected_packages, priority, resolveGlobs(value));
+        }, nullptr, true
     );
 
     owner.optBinds().add("username", username);

libdnf/conf/ConfigRepo.cpp

@@ -145,7 +145,13 @@ ConfigRepo::Impl::Impl(Config & owner, ConfigMain & mainConfig)
     owner.optBinds().add("proxy_auth_method", proxy_auth_method);
     owner.optBinds().add("username", username);
     owner.optBinds().add("password", password);
-    owner.optBinds().add("protected_packages", protected_packages);
+
+    owner.optBinds().add("protected_packages", protected_packages,
+        [&](Option::Priority priority, const std::string & value){
+            optionTListAppend(protected_packages, priority, value);
+        }, nullptr, true
+    );
+
     owner.optBinds().add("gpgcheck", gpgcheck);
     owner.optBinds().add("repo_gpgcheck", repo_gpgcheck);
     owner.optBinds().add("enablegroups", enablegroups);