New MemberMapping

I know @gavinking is not super keen on unsealing `ResultSetMapping` - #887.

As mentioned there, there are a few things in Hibernate's legacy dynamic ResultSet mapping API for native SQL that we cannot express in `ResultSetMapping`. The biggest one is the notion of mapping a join fetch (to-one).

As an alternative to #887, how about adding the ability to map fetched to-one associations -

```java
public sealed interface MemberMapping
permits FieldMapping, EmbeddedMapping, ToOneMapping {
}

public record ToOneMapping
(Class containerType, String alias,
String name, Class toOneType,
ColumnMapping[] fkColumns,
...) implements MemberMapping {
}
```

E.g.

```java
var sqlString = """
select ...,
b.title,
p.name,
b.publisher_fk,
b.id,
p.id
from books b
join publishers p on b.publisher_fk = p.id
""";

var publisher = toOne(Book_.publisher,
"p",
members(
field("name", ... ),
...
)
);
var bookMapping = entity(Book.class,
"b",
members(
field("title" ),
publisher,
...
)
);

em.createNativeQuery( sqlString, bookMapping );
```

Start by reading the existing ResultSetMapping API and the discussion in #887, then compare the legacy dynamic ResultSet mapping behavior described here. Done means agreeing on and implementing a supported MemberMapping design that can express fetched to-one associations for native SQL queries.

Written by the indexing model from the issue text.