Add a method to get the SQL query

[marcominerva]

In some scenario it is useful to just get the query, and then let the user decide what to do with it. So, the following method must be added to DatabaseGptClient:

public interface IDatabaseGptClient : IDisposable
{
    Task<string> GetNaturalLanguageQueryAsync(Guid sessionId, string question, CancellationToken cancellationToken = default);
}