[FEA] chunked_pack should allow variable sized spans in the next call

As brought up here https://github.com/rapidsai/cudf/pull/13260#discussion_r1189086902, `chunked_pack` requires that it be configured with a single size for the user's bounce buffer, and further requires the same size of device memory made available in every call to `chunked_pack::next`.

This is overkill in the case where we have actually very little work to be done (the source table is small). So the proposal is to allow for a `get_next_size()` api that lets us know the size of buffer that should be made available to `next`