Make X509_ATTRIBUTE accessor functions const-correct

Make X509_ATTRIBUTE accessor functions const-correct

The X509_ATTRIBUTE accessor functions were not const-correct, preventing
callers from usefully interacting with a const X509_ATTRIBUTE pointer.

Update the following functions to accept const X509_ATTRIBUTE * and
return const pointers where appropriate:

- X509_ATTRIBUTE_get0_object: returns const ASN1_OBJECT *
- X509_ATTRIBUTE_get0_type: returns const ASN1_TYPE *
- X509_ATTRIBUTE_get0_data: returns const void *

Also update dependent PKCS12 functions:
- PKCS12_get_attr_gen: returns const ASN1_TYPE *
- PKCS12_get_attr: returns const ASN1_TYPE * (deprecated)
- PKCS8_get_attr: returns const ASN1_TYPE *

Update all callers to use const pointers for the return values.

Fixes #29811

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 14:46:26 2026
(Merged from https://github.com/openssl/openssl/pull/29813)