[FEATURE]: discriminator column and custom type

### Describe what you want

Hi,

I have a use case similar to the following one, I have a discriminator column that influence the content of a json field

```typescript

export const Section = mysqlTable(
'BillingDetail',
{
id: varchar('id', 200)
.primaryKey()
.notNull(),
kind: varchar('kind', {
length: DEFAULT_VARCHAR_LENGTH,
enum: [
'creditCard',
'bankAccount'
],
}).notNull(),
data: customType<{
data: BillingData;
}>({
toDriver: value => JSON.stringify(value),
fromDriver: value => value as BillingData,
dataType: () => 'json',
})('data').notNull(),

type BillingData = {
expiryMonth?: number;
expiryYear?: number;
cardType?: string;

bankName?: string;
swift?: string;
}

```

the type `BillingData` contains all the field that can be contained in `data`

I would like to have something like

```typescript
const BillingDetail = { id: string; } &
(
{ kind: 'creditCard'; data: { expiryMonth: number; expiryYear: number; cardType: string} }
| 
{ kind: 'bankAccount'; data: { bankName: string; swift: string} }
)
```
as result.

But I can’t use my discriminator column (`kind`) in my `customType` to specify more precisely the data

It would be great to have a way to refine types based on other columns (notably for json columns), to be able define a schema as a union