[Validation] Prevent "all string literal" unions and suggest to use enums instead

Instead of:

```ts
export type ManagedBy =
  | 'Index Lifecycle Management'
  | 'Data stream lifecycle'
  | 'Unmanaged'
```

validation should suggest:

```ts
enum ManagedBy {
  'Index Lifecycle Management',
  'Data stream lifecycle',
  'Unmanaged'
}
```

cc @swallez 