The paginator logic was built when go-memdb iterators would return items
ordered lexicographically by their ID prefixes, but #12054 added the
option for some tables to return results ordered by their `CreateIndex`
instead, which invalidated the previous paginator assumption.
The iterator used for pagination must still return results in some order
so that the paginator can properly handle requests where the next_token
value is not present in the results anymore (e.g., the eval was GC'ed).
In these situations, the paginator will start the returned page in the
first element right after where the requested token should've been.
This commit moves the logic to generate pagination tokens from the
elements being paginated to the iterator itself so that callers can have
more control over the token format to make sure they are properly
ordered and stable.
It also allows configuring the paginator as being ordered in ascending
or descending order, which is relevant when looking for a token that may
not be present anymore.
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