Allow multiple sort params

[timsmelik]

Spring data Pageable supports sorting on multiple dimensions.

On the client side, we can do this by adding multiple sort params, e.g. /api/users?sort=age,desc&sort=name,asc. The PagingClient doesn't support this however, because it expects the sort param to be a single string. We should also allow users to pass in a list of strings.

As part of this change, we could also export the Pagable type for reuse.