Add name() static method to VectorSerde classes (facebookincubator#16757)

Summary:
Pull Request resolved: facebookincubator#16757

Add static name() method to PrestoVectorSerde, CompactRowVectorSerde,
and UnsafeRowVectorSerde to provide a compile-time serde name accessor.
This allows callers to reference serde names without hardcoding strings.

This is extracted from D96046667 as a minimal API addition to unblock
axiom migration (D96254285) while the full refactor is reviewed.

Reviewed By: xiaoxmeng

Differential Revision: D96412951

Author: han-yan01

velox/serializers/CompactRowSerializer.h

@@ -22,7 +22,12 @@ namespace facebook::velox::serializer {
 
 class CompactRowVectorSerde : public VectorSerde {
  public:
-  CompactRowVectorSerde() : VectorSerde("CompactRow") {}
+  CompactRowVectorSerde() : VectorSerde(name()) {}
+
+  static const std::string& name() {
+    static const std::string kName{"CompactRow"};
+    return kName;
+  }
 
   void estimateSerializedSize(
       const row::CompactRow* compactRow,

velox/serializers/PrestoSerializer.h

@@ -88,7 +88,12 @@ class PrestoVectorSerde : public VectorSerde {
     bool preserveEncodings{false};
   };
 
-  PrestoVectorSerde() : VectorSerde("Presto") {}
+  PrestoVectorSerde() : VectorSerde(name()) {}
+
+  static const std::string& name() {
+    static const std::string kName{"Presto"};
+    return kName;
+  }
 
   /// Adds the serialized sizes of the rows of 'vector' in 'ranges[i]' to
   /// '*sizes[i]'.

velox/serializers/UnsafeRowSerializer.h

@@ -21,7 +21,12 @@ namespace facebook::velox::serializer::spark {
 
 class UnsafeRowVectorSerde : public VectorSerde {
  public:
-  UnsafeRowVectorSerde() : VectorSerde("UnsafeRow") {}
+  UnsafeRowVectorSerde() : VectorSerde(name()) {}
+
+  static const std::string& name() {
+    static const std::string kName{"UnsafeRow"};
+    return kName;
+  }
 
   void estimateSerializedSize(
       const row::UnsafeRowFast* unsafeRow,