Perf: improve batching for TPH

[AndriySvyryd]

We could create larger batches by combining operations on entities of base and derived types.

[divega]

This issue maps to a TODO in SqlServerUpdateSqlGenerator.AppendBulkInsertOperation(...)

[roji]

@AndriySvyryd do you remember what exactly this refers to, and whether it's still relevant? At the very least derived and base inserts into TPH seem to be batched already (code below).

Code sample

await using var ctx = new BlogContext();
await ctx.Database.EnsureDeletedAsync();
await ctx.Database.EnsureCreatedAsync();

ctx.Add(new Derived { BaseProp = "Base", DerivedProp = "Derived" });
ctx.Add(new Base { BaseProp = "Base2" });
await ctx.SaveChangesAsync();

public class BlogContext : DbContext
{
    public DbSet<Base> Base { get; set; }
    public DbSet<Derived> Derived { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        => optionsBuilder
            .UseSqlServer(@"Server=localhost;Database=test;User=SA;Password=Abcd5678;Connect Timeout=60;ConnectRetryCount=0;Encrypt=false",
                o => o.MinBatchSize(1))
            .LogTo(Console.WriteLine, LogLevel.Information)
            .EnableSensitiveDataLogging();
}

public class Base
{
    public int Id { get; set; }
    public string BaseProp { get; set; }
}

public class Derived : Base
{
    public string DerivedProp { get; set; }
}