Propagate const-correctness to PKCS7, CMS, and X509 attribute functions

Propagate const-correctness to PKCS7, CMS, and X509 attribute functions

Following the const-correctness changes to X509_ATTRIBUTE accessor
functions, update all dependent functions to also return const pointers:

PKCS7 functions:
- PKCS7_get_attribute: returns const ASN1_TYPE *
- PKCS7_get_signed_attribute: returns const ASN1_TYPE *
- PKCS7_digest_from_attributes: returns const ASN1_OCTET_STRING *

X509 functions:
- X509at_get0_data_by_OBJ: returns const void *

CMS functions:
- CMS_signed_get0_data_by_OBJ: returns const void *
- CMS_unsigned_get0_data_by_OBJ: returns const void *

Update all callers to use const-qualified pointers for return values.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
MergeDate: Fri Feb 13 14:46:28 2026
(Merged from https://github.com/openssl/openssl/pull/29813)