Constify X509_get_ext() and friends..

Constify X509_get_ext() and friends..

These all took const, but returned non const, they should return const.

This then triggers constifying of a whole class of get_ext() functions.

Part of #28654 and #29117
Fixes: openssl/project#1779
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
MergeDate: Tue Feb 24 18:53:25 2026
(Merged from https://github.com/openssl/openssl/pull/29465)