Enable reading explicit row number column from Prism connector

Summary:
Also add the missing implementation for
`FlatMapAsStructColumnReader::estimateMaterializedSize`.

Differential Revision: D63151114

velox/dwio/common/ColumnSelector.cpp

@@ -344,7 +344,7 @@ std::shared_ptr<ColumnSelector> ColumnSelector::fromScanSpec(
     const RowTypePtr& rowType) {
   std::vector<std::string> columnNames;
   for (auto& child : spec.children()) {
-    if (child->isConstant()) {
+    if (child->isConstant() || child->isExplicitRowNumber()) {
       continue;
     }
     std::string name = child->fieldName();

velox/exec/tests/TableScanTest.cpp

@@ -5090,6 +5090,10 @@ TEST_F(TableScanTest, rowNumberInRemainingFilter) {
   auto file = TempFilePath::create();
   writeToFile(file->getPath(), {vector});
   auto outputType = ROW({"c1"}, {BIGINT()});
+  // We should not need r1 in table schema for production code; this is just to
+  // infer type of r1 in remaining filter expression parser.  We probably need
+  // to find a way to specify the type of r1 without adding it to table schema
+  // to test this case.  For now it's essentially untested.
   auto schema = ROW({"c1", "r1"}, {BIGINT(), BIGINT()});
   auto plan = PlanBuilder()
                   .startTableScan()