Add accessor functions for "mbedtls_x509_crt" data structure for version 3.0

[bbesharat]

Suggested enhancement

Provide a function to access next field of mbedtls_x509_crt structure.
struct mbedtls_x509_crt * mbedtls_x509_crt_get_next();

Justification

I pass next certificate in the CA-chain from mbedtls_x509_crt structure by accessing "next" field of this structure to "mbedtls_ssl_conf_ca_chain" function to set server certificate chain as follow:
/* Set server certificate chain /
mbedtls_ssl_config conf; / has been initialized and setup before this call /
mbedtls_x509_crt srvcert; / has been initialized and setup before this call */
mbedtls_ssl_conf_ca_chain(&conf, srvcert.next, NULL);

Mbed TLS needs this because
Who ever tries to access any member of mbedtls_x509_crt structure would need accessor function to access its members in MbedTLS v3.x

[gabor-mezei-arm]

This field is made public, no longer need an accessor function. Close as a duplicate of #5019.