Move tar detection and compression logic to common.

Move tar detection and compression logic to common.

Consolidate tar archive identification and compression-type detection
logic into a shared location. Currently used by pg_basebackup and
pg_verifybackup, this functionality is also required for upcoming
pg_waldump enhancements.

This change promotes code reuse and simplifies maintenance across
frontend tools.

Author: Amul Sul <sulamul@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
discussion: https://postgr.es/m/CAAJ_b94bqdWN3h2J-PzzzQ2Npbwct5ZQHggn_QoYGhC2rn-=WQ@mail.gmail.com