crypto/x509-utils: Add helper functions for DIAG 320 subcode 2

Introduce new helper functions to extract certificate metadata:

qcrypto_x509_check_cert_times() - validates the certificate's validity period against the current time
qcrypto_x509_get_cert_key_id() - extracts the key ID from the certificate
qcrypto_x509_check_ecc_curve_p521() - determines the ECC public key algorithm uses P-521 curve

These functions provide support for metadata extraction and validity checking
for X.509 certificates.

Signed-off-by: Zhuoying Cai <zycai@linux.ibm.com>
Acked-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Farhan Ali <alifm@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260730214624.2328883-9-zycai@linux.ibm.com
Signed-off-by: Eric Farman <farman@linux.ibm.com>