Filter and sort known fields by default

Sometimes, we embed structs that we don't have access to edit, but we still want to be able to filter and sort by them. For example:
```go
type User struct {
gorm.Model
// ...
}
```
Therefore, I want to add support for filtering and sorting known fields. These fields can be part of the configuration of the parser with some default values.

The issue is about extending the RQL parser to filter and sort by known fields from embedded structs like gorm.Model. Start by examining the parser configuration and how fields are currently discovered. Look for where field validation and query building happens. The goal is to allow pre-defined fields in the config and have them work in filter/sort operations.

Scritto dal modello di indicizzazione a partire dal testo della issue.