Add raw buffer holding SubjectAlternativeName ext to CRT structure

This is analogous to a previous commit for the `ExtendedKeyUsage`
extension: We aim at not using dynamically allocated linked lists
to represent the components of the `SubjectAlternativeName` extension,
but to traverse the raw ASN.1 data when needed.

This commit adds a field to `mbedtls_x509_crt` containing the raw
ASN.1 buffer bounds of the `SubjectAlternativeNames` extension.

include/mbedtls/x509_crt.h

@@ -80,6 +80,7 @@ typedef struct mbedtls_x509_crt
     mbedtls_x509_buf subject_id;        /**< Optional X.509 v2/v3 subject unique identifier. */
     mbedtls_x509_buf v3_ext;            /**< Optional X.509 v3 extensions.  */
     mbedtls_x509_sequence subject_alt_names;    /**< Optional list of Subject Alternative Names (Only dNSName supported). */
+    mbedtls_x509_buf_raw subject_alt_raw; /**< Raw data for SubjectAlternativeNames extension. */
 
     int ext_types;              /**< Bit string containing detected and parsed extensions */
     int ca_istrue;              /**< Optional Basic Constraint extension value: 1 if this certificate belongs to a CA, 0 otherwise. */