allow for iceberg element-ids take 2

Iceberg manifests need to be able to write their JSONified Avro schemas
in the Avro headers for certain Iceberg clients (e.g. PyIceberg) to be
able to read them.

Commit 966d457 was a stab at allowing
this by hacking this custom field into the AST, but it did so as a
string, instead of a long. This isn't exactly what other libraries
output, but is sufficient for PyIceberg. To avoid surprises down the
line, this quick hack switches us to serialize the long.

(cherry picked from commit b2b82a4)

lang/c++/impl/Compiler.cc

@@ -439,7 +439,7 @@ static NodePtr makeArrayNode(const Entity &e, const Object &m,
         asSingleAttribute(makeNode(it->second, st, ns)));
     NodePtr node = NodePtr(nodePointer);
     if (containsField(m, "element-id")) {
-        nodePointer->elementId_ = getStringField(e, m, "element-id");
+        nodePointer->elementId_ = getLongField(e, m, "element-id");
     }
     if (containsField(m, "doc")) {
         node->setDoc(getDocField(e, m));

lang/c++/impl/NodeImpl.cc

@@ -582,9 +582,9 @@ void NodeArray::printJson(std::ostream &os, size_t depth) const {
         logicalType().printJson(os);
         os << ",\n";
     }
-    if (!elementId_.empty()) {
-        os << indent(depth + 1) << R"("element-id": ")"
-           << escape(elementId_) << "\",\n";
+    if (elementId_.has_value()) {
+        os << indent(depth + 1) << "\"element-id\": "
+           << *elementId_ << ",\n";
     }
     os << indent(depth + 1) << "\"items\": ";
     leafAttributes_.get()->printJson(os, depth + 1);

lang/c++/include/avro/NodeImpl.hh

@@ -26,6 +26,7 @@
 #include <iostream>
 #include <limits>
 #include <memory>
+#include <optional>
 #include <set>
 #include <sstream>
 #include <utility>
@@ -390,7 +391,7 @@ public:
 
     void printDefaultToJson(const GenericDatum &g, std::ostream &os, size_t depth) const override;
 
-    std::string elementId_;
+    std::optional<int64_t> elementId_;
 };
 
 class AVRO_DECL NodeMap : public NodeImplMap {

lang/c++/test/SchemaTests.cc

@@ -198,7 +198,7 @@ const char *roundTripSchemas[] = {
     R"({"type":"array","items":"long"})",
     "{\"type\":\"array\",\"items\":{\"type\":\"enum\","
     "\"name\":\"Test\",\"symbols\":[\"A\",\"B\"]}}",
-    R"({"type":"array","element-id":"testElemId","items":"long"})",
+    R"({"type":"array","element-id":42,"items":"long"})",
 
     // Map
     R"({"type":"map","values":"long"})",