Generate ENUM fields as python `Enum` types or string `Literal` types

[sheinbergon]

Things to check first

• I have searched the existing issues and didn't find my feature already requested there

Feature description

Today, enum fields are generated as str columns. I'd like bind them to generated native python Enum or string Literals instead.

Use case

This would greatly ease the use of native RDBMs enums in the code, providing tough constraints (python Enum) or soft hints (Literal), and will help prevent human errors when developing against generated sqlacodegen models

[agronholm]

The problem with Literal is that you have to pass it a string literal, which is a problem if your data comes from a dynamic source.

[agronholm]

Generating enum classes might be a good feature, but I'd like to have a way to opt out of that.