Comparing two fields

Typetta should be able to accept fields comparison in the filter:

proposal 1:
```typescript
// field and operator are type safe
await user.findAll({ filter: { updatedAt: { compare: { field: 'createdAt', operator: 'gt' } } } })
```

proposal 2
```typescript
// compare is type safe
await user.findAll({ filter: { updatedAt: { compare: '> createdAt' } } })
```