CallFunction: add a table row iterator function

[fw2568]

Getting values from table rows is currently quite complicated:

using (var context = new RfcContext(ConnFunc))
{
    await context.CallFunction("BAPI_COMPANYCODE_GETLIST",
        Output: func => func.BindAsync(f =>
            from companyTable in f.GetTable("COMPANYCODE_LIST")

            from row in companyTable.Rows.Map(s =>
              from code in s.GetField<string>("COMP_CODE")
              from name in s.GetField<string>("COMP_NAME")
              select (code, name)).Traverse(l => l)

        select row))
    .ToAsync().Match(
        r =>
            {
                foreach (var (code, name) in r)
                {
                    Console.WriteLine($"{code}\t{name}");
                }
            },
        l=> Console.WriteLine($"Error: {l.Message}"));
}

We should add a "MapStructure" method to simplify this:

using (var context = new RfcContext(ConnFunc))
{
    await context.CallFunction("BAPI_COMPANYCODE_GETLIST",
        Output: func => func.BindAsync(f =>
            from companyTable in f.GetTable("COMPANYCODE_LIST")

            from row in companyTable.MapStructure(s =>
              from code in s.GetField<string>("COMP_CODE")
              from name in s.GetField<string>("COMP_NAME")
              select (code, name))

        select row))
    .ToAsync().Match(
        r =>
            {
                foreach (var (code, name) in r)
                {
                    Console.WriteLine($"{code}\t{name}");
                }
            },
        l=> Console.WriteLine($"Error: {l.Message}"));
}