Add optional compression method to SP-GiST

Add optional compression method to SP-GiST

Patch allows to have different types of column and value stored in leaf tuples
of SP-GiST. The main application of feature is to transform complex column type
to simple indexed type or for truncating too long value, transformation could
be lossy.  Simple example: polygons are converted to their bounding boxes,
this opclass follows.

Authors: me, Heikki Linnakangas, Alexander Korotkov, Nikita Glukhov
Reviewed-By: all authors + Darafei Praliaskouski
Discussions:
https://www.postgresql.org/message-id/5447B3FF.2080406@sigaev.ru
https://www.postgresql.org/message-id/flat/54907069.1030506@sigaev.ru#54907069.1030506@sigaev.ru