improve expression codegen support

[zhouyuan]

Is your feature request related to a problem or challenge? Please describe what you are trying to do. below expression codegen are missing

• [ ] lpad
• [ ] hex
• [ ] conv
• [ ] lower
• [ ] date_add

Describe the solution you'd like improve expression codegen to support these expressions https://github.com/oap-project/gazelle_plugin/blob/main/native-sql-engine/cpp/src/codegen/arrow_compute/ext/expression_codegen_visitor.cc

Describe alternatives you've considered

N/A

Additional context N/A