Implement CURRENT_TIMESTAMP support for datetime

[lauxjpn]

We did already implement this for timestamp columns.

For the following table:

CREATE TABLE IF NOT EXISTS `modTest`(
  `id` INT NOT NULL AUTO_INCREMENT,
  `modify_date1` DATETIME NULL,
  `modify_date2` DATETIME NOT NULL,
  `modify_date3` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `modify_date4` DATETIME NULL ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

The following model and definition is now being scaffolded:

public partial class ModTest
{
    public int Id { get; set; }
    public DateTime? ModifyDate1 { get; set; }
    public DateTime ModifyDate2 { get; set; }
    public DateTime ModifyDate3 { get; set; }
    public DateTime? ModifyDate4 { get; set; }
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<ModTest>(entity =>
    {
        entity.ToTable("modTest");

        entity.Property(e => e.Id)
            .HasColumnName("id")
            .HasColumnType("int(11)");

        entity.Property(e => e.ModifyDate1)
            .HasColumnName("modify_date1")
            .HasColumnType("datetime");

        entity.Property(e => e.ModifyDate2)
            .HasColumnName("modify_date2")
            .HasColumnType("datetime");

        entity.Property(e => e.ModifyDate3)
            .HasColumnName("modify_date3")
            .HasColumnType("datetime")
            .HasDefaultValueSql("CURRENT_TIMESTAMP");

        entity.Property(e => e.ModifyDate4)
            .HasColumnName("modify_date4")
            .HasColumnType("datetime")
            .ValueGeneratedOnAddOrUpdate();
    });

    OnModelCreatingPartial(modelBuilder);
}

Fixes #958