Design the udf class of feature encoding methods

Design the basic class of feature encoding.

The issue is about designing a class for feature encoding methods in OpenMLDB, a machine learning database. Start by exploring the existing codebase structure for feature encoding or UDF (User-Defined Function) implementations. Look for related classes or modules in the source code to understand the current design patterns. Determine what 'basic class of feature encoding' entails by reviewing documentation or similar features in the project.

索引モデルが issue の本文から書いたものです。