rename variant to implementation

Author: chaojun-zhang

include/substrait/function/Extension.h

@@ -40,13 +40,13 @@ class Extension {
   static std::shared_ptr<Extension> load(
       const std::vector<std::string>& extensionFiles);
 
-  /// Add a scalar function variant.
+  /// Add a scalar function implementation.
   void addScalarFunctionImpl(const FunctionImplementationPtr& functionImpl);
 
-  /// Add a aggregate function variant.
+  /// Add a aggregate function implementation.
   void addAggregateFunctionImpl(const FunctionImplementationPtr& functionImpl);
 
-  /// Add a window function variant.
+  /// Add a window function implementation.
   void addWindowFunctionImpl(const FunctionImplementationPtr& functionImpl);
 
   /// Add a type variant.