Support btree_gin extension

The btree_gin extension is useful to create GIN indexes where some expressions are using Range or Equality.

Of course they are less performent then a BTREE if no Similarity is needed. But if there is a mix of Similarity and (Range or Equality), then having a single index instead of two AND-ed indexes may be better.

Also BTREE index has a size limit: https://www.postgresql.org/message-id/25511356.post@talk.nabble.com but not GIN.