Unreachable concrete column schema nodes

[paulourio]

In ZetaSQL, ASTColumnSchema is an abstract class, and its extensions are

• ASTArrayColumnSchema
• ASTInferredTypeColumnSchema
• ASTSimpleColumnSchema
• ASTStructColumnSchema

However, in Go bindings, we have a struct ast.ColumnSchemaNode and ast.Nodes's of kind ast.ArrayColumnSchema, ast.InferredTypeColumnSchema, ast.StructColumnSchema, and ast.StructColumnSchema are mapped to *ast.ColumnSchemaNode.
Effectively, I cannot reach any of ast.ArrayColumnSchemaNode, ast.InferredTypeColumnSchemaNode, ast.StructColumnSchemaNode, or ast.ArrayColumnSchemaNode by walking with Child() methods.