Filter by the concatenation of two columns

## Suggestion

It would be nice to be able to filter a Model by the concatenation of two columns.
I mean, to be able to replicate this type of SQL query:
```sql
SELECT * FROM User WHERE CONCAT(name, ' ', lastName) LIKE '%PARTIAL FULLNAME%'
```

## Use Cases

The most common example is filtering a User model by its full name when it's stored in two columns (name, lastName).
Currently, the only way this is possible (AFAIK) is by sending the RAW query, which makes it almost incompatible with adding more filters.

## Examples
Table example:
| id | name | lastName |
|----|---------|----------|
| 1 | Jackson | Bell |
| 2 | Sarah | Parker |
| 3 | Mason | Brown |

Example of one (possible) implementation of filter object:
```json
{
"limit": 10,
"offset": 0,
"where": {
"leavingDate": null,
"concat": {
"eq": "%on B%",
"separator": " ",
"columns": [
"name",
"lastName"
]
}
}
}
```

Should return:
| id | name | lastName |
|----|---------|----------|
| 1 | Jackson | Bell |
| 3 | Mason | Brown |

## Acceptance criteria

TBD - will be filled by the team.

Start by reviewing the proposed `where.concat` filter object and how repository filters currently handle raw queries. Clarify the acceptance criteria with the team, including supported columns, separators, comparison operators, and database behavior. Done means concatenation filtering works alongside other filters and has documented verification.

Written by the indexing model from the issue text.