Avoid sorts for descending ORDER BY on lookup join index columns

[msirek]

Is your feature request related to a problem? Please describe.
#84689 improves the number of cases a sort is avoided for lookup join queries with ORDER BY on the lookup index columns, but does not handle descending index columns

Example:

CREATE TABLE xyz (x INT, y INT, z INT, PRIMARY KEY(x, y, z));
CREATE TABLE uvw (u INT, v INT, w INT, PRIMARY KEY(u, v, w));

The following avoids a sort:

SELECT * FROM xyz INNER LOOKUP JOIN uvw ON x = u AND y = v ORDER BY x, y, z, u, v, w;

But if the PK of table uvw is (u, v, w DESC) and we have ORDER BY x, y, z, u, v, w DESC; This should avoid a sort.
A reverse scan is not required so I believe lookup join should be able to support it.

Describe alternatives you've considered
None

Jira issue: CRDB-19769

[DrewKimball]

In order to make this happen, the joinreader will need to be able to issue reverse scan requests. We'll also have to change the streamer codepath to make sure the scan iteration in Enqueue respects the index ordering, at least when the ordering has to be preserved. Assuming the non-streamer codepath is still around whenever we get around to this, that will have to be changed as well.