add reverse pagination support and new cursor format

This release changes the pagination cursor format from "", where is a UUID, to "id:", with again represented as a UUID.

In addition to the existing "starting_after" and "limit" options, which take a cursor value and page size respectively, we also introduce the following pagination options:

ending_before - takes a cursor in the same "id:" format, resulting in the query returning "limit" results before the cursor

sort_order - either "ascending", the default, or "descending". If "ascending", results are returned in ascending order from the start, or if "descending", they are returned in descending order from the end.