Support non-btree indexes in get_actual_variable_range()

Support non-btree indexes in get_actual_variable_range()

This was previously not supported because the btree strategy numbers
were hardcoded.  Now we can support this for any index that has the
required strategy mapping support and the required operators.

If an index scan used for get_actual_variable_range() requires
recheck, we now just ignore it instead of erroring out.  With btree we
knew this couldn't happen, but now it might.

Author: Mark Dilger <[email protected]>
Co-authored-by: Peter Eisentraut <[email protected]>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com