Pivot Measures Order Used in Pivot Exports (#49367)

* Pivot Measures Order Used in Pivot Exports

Fixes #48442

A pivot table can have any number of measures, and the user can order these by dragging in the UI. Before this PR,
that order was ignored and measures would alway be in index order, which is confusing for any user who needs the
measures to be displayed in a particular order, especially if they've re-ordered them in the pivot viz settings UI.

A test has been added to check that measure order is used.

A few minor changes to the pivot qp and post-processor
 - measure indices are looked up in the pivot qp and added in viz-settings order
 - the pivot measures are only added if the qp has not already added them.
 - pivot-opts Malli spec has been made in the namespace, adjusted to allow `nil` as a valid pivot-opts output, and
 used in relevant functions

* address review points.

* add a rows order test