src: back snapshot I/O with a std::vector sink

PR-URL: #46463
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>

Author: joyeecheung