Use a FilterFids request for vector file writer rather than fetching

all features and then testing the feature id when saving selected
features

For providers such as Postgres this is orders of magnitude faster.
Eg, saving 6 selected features from a 800k PostGIS table now
takes < 1 second as opposed to ~33 seconds

Fix #8334

src/core/qgsvectorfilewriter.cpp

@@ -1982,10 +1982,10 @@ QgsVectorFileWriter::WriterError QgsVectorFileWriter::writeAsVectorFormat( QgsVe
     req.setFlags( QgsFeatureRequest::NoGeometry );
   }
   req.setSubsetOfAttributes( allAttr );
+  if ( onlySelected )
+    req.setFilterFids( layer->selectedFeaturesIds() );
   QgsFeatureIterator fit = layer->getFeatures( req );
 
-  const QgsFeatureIds& ids = layer->selectedFeaturesIds();
-
   //create symbol table if needed
   if ( writer->symbologyExport() != NoSymbology )
   {
@@ -2027,9 +2027,6 @@ QgsVectorFileWriter::WriterError QgsVectorFileWriter::writeAsVectorFormat( QgsVe
   // write all features
   while ( fit.nextFeature( fet ) )
   {
-    if ( onlySelected && !ids.contains( fet.id() ) )
-      continue;
-
     if ( shallTransform )
     {
       try