Make it possible to let plasmoids to load scripts outside the plasma package

Adds a X-Plasma-Requires property for the plasmoid metadata files that will
specify the features.
Furthermore, adds a feature called ExternalScripts so that a plasmoid can
request to load files that are outside of the import directories.

REVIEW: 122595

src/plasma/data/servicetypes/plasma-applet.desktop

@@ -66,3 +66,6 @@ Type=QStringList
 
 [PropertyDef::X-Plasma-StandAloneApp]
 Type=bool
+
+[PropertyDef::X-Plasma-RequiredExtensions]
+Type=QStringList

src/plasmaquick/packageurlinterceptor.cpp

@@ -115,7 +115,7 @@ QUrl PackageUrlInterceptor::intercept(const QUrl &path, QQmlAbstractUrlIntercept
 
         //forbid to load random absolute paths
     } else {
-        if (m_package.allowExternalPaths()) {
+        if (m_package.allowExternalPaths() || m_package.metadata().property("X-Plasma-RequiredExtensions").toString().contains(QStringLiteral("ExternalScripts"))) {
             return path;
         }