Add date_bin function

Add date_bin function

Similar to date_trunc, but allows binning by an arbitrary interval
rather than just full units.

Author: John Naylor <john.naylor@enterprisedb.com>
Reviewed-by: David Fetter <david@fetter.org>
Reviewed-by: Isaac Morland <isaac.morland@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Artur Zakirov <zaartur@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CACPNZCt4buQFRgy6DyjuZS-2aPDpccRkrJBmgUfwYc1KiaXYxg@mail.gmail.com