Implement cost_of_filesort()

The sort length is extracted similarly to how sortlength() function does
it. The function makes use of filesort_use_addons function to compute
the length of addon fields. Finally, by calling compute_sort_costs we
get the fastest_sort possible.

Other changes:
* Sort_param::using_addon_fields() assumes addon fields are already
  allocated. This makes the use of Sort_param unusable for
  compute_sort_costs *if* we don't want to allocate addon fields.

  As a preliminary fix, pass "with_addon_fields" as bool value to
  compute_sort_costs() and make the internal functions use that value
  instead of Sort_param::using_addon_fields() method.

  The ideal fix would be to define a "leaner" struct with only the
  necessary members, but this can be done as a separate commit.

Reviewer: Monty

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