Support enums in gentool

[diogox]

Describe the feature

On Postgres, we can define:

CREATE TYPE fruit AS ENUM ('orange', 'pineapple', 'peach');

Other tools that generate Go code from DB schemas, such as sqlboiler, will generate the following:

type Fruit string

// Enum values for Fruit
const (
	FruitOrange       Fruit = "orange"
	FruitPineapple    Fruit = "pineapple"
	FruitPeach        Fruit = "peach"
)

I was wondering if we can have the same feature in gentool.

Motivation

Without this code generation, we run the risk of checking against the wrong string values (especially when adding/removing them from the schema).

Having this generated, would allow us to check confidently against the schema like so:

if result.Fruit == model.FruitOrange {
  // TODO
}

Related Issues

#1033 - contrary to this ticket, I'd like the gentool to generate the constants for me based on the schema it already has. Not map them to manually written ones.