Move backup-related code to xlogbackup.c/.h

When Who What
2023-03-24 03:31:57 Greg Stark (stark) Closed in commitfest 2023-03 with status: Rejected
2023-01-31 16:18:30 vigneshwaran C (vignesh.postgres) Closed in commitfest 2023-01 with status: Moved to next CF
2022-12-12 13:22:23 Ian Barwick (barwick) Closed in commitfest 2022-11 with status: Moved to next CF
2022-10-04 22:54:27 Nathan Bossart (bossartn) Added bossartn as reviewer
2022-09-29 06:17:04 Bharath Rupireddy (bharathrupireddy) Changed authors to Bharath Rupireddy (bharathrupireddy)
2022-09-29 06:16:52 Bharath Rupireddy (bharathrupireddy) Attached mail thread CALj2ACU=1+7XB2kq89yY9esn0P-Rr1fEBYAHu1LoTCpEaqWOKQ@mail.gmail.com
2022-09-29 06:16:52 Bharath Rupireddy (bharathrupireddy) Created patch record