pg_upgrade: Add --set-char-signedness to set the default char signedness of new cluster.
This change adds a new option --set-char-signedness to pg_upgrade. It
enables user to set arbitrary signedness during pg_upgrade. This helps
cases where user who knew they copied the v17 source cluster from
x86 (signedness=true) to ARM (signedness=false) can pg_upgrade
properly without the prerequisite of acquiring an x86 VM.
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/
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