feat: add support for SAN (DNS names and IP addresses).Resolves #458

[longxiucai]

Introduces a new gauge metric x509_cert_san to expose Subject Alternative Names (SANs) from X.509 certificates, with detailed metadata labels for enhanced monitoring and troubleshooting.

Key features:

• Tracks both DNS names and IP addresses via SAN_type label ("dns" or "address")
• Includes SAN_value label to store the actual DNS/IP entry
• Adds base labels:filename,filepath,issuer_CN,subject_CN,serial_number, etc.

Example metric output:

# HELP x509_cert_san Indicates the Subject Alternative Names (DNS or IP) present in the certificate
# TYPE x509_cert_san gauge
x509_cert_san{SAN_type="address",SAN_value="10.103.97.2",filename="apiserver.crt",filepath="/file/go/src/test/apiserver.crt",issuer_CN="kubernetes",serial_number="8451564427851205067",subject_CN="kube-apiserver"} 1
...
x509_cert_san{SAN_type="address",SAN_value="10.77.0.1",filename="apiserver.crt",filepath="/file/go/src/test/apiserver.crt",issuer_CN="kubernetes",serial_number="8451564427851205067",subject_CN="kube-apiserver"} 1
x509_cert_san{SAN_type="address",SAN_value="127.0.0.1",filename="apiserver.crt",filepath="/file/go/src/test/apiserver.crt",issuer_CN="kubernetes",serial_number="8451564427851205067",subject_CN="kube-apiserver"} 1
...
x509_cert_san{SAN_type="dns",SAN_value="kubernetes.default.svc.cluster.local",filename="apiserver.crt",filepath="/file/go/src/test/apiserver.crt",issuer_CN="kubernetes",serial_number="8451564427851205067",subject_CN="kube-apiserver"} 1
x509_cert_san{SAN_type="dns",SAN_value="localhost",filename="apiserver.crt",filepath="/file/go/src/test/apiserver.crt",issuer_CN="kubernetes",serial_number="8451564427851205067",subject_CN="kube-apiserver"} 1
x509_cert_san{SAN_type="dns",SAN_value="node1",filename="apiserver.crt",filepath="/file/go/src/test/apiserver.crt",issuer_CN="kubernetes",serial_number="8451564427851205067",subject_CN="kube-apiserver"} 1

This implementation enables granular filtering of SAN entries (e.g., finding all certificates with a specific DNS name) while preserving full certificate context for cross-referencing with existing metrics.