`GET_CERTIFICATE` public API

[steven-bellock]

From #1655.

There are currently three public APIs to fetch a certificate chain. The length parameter is no longer needed now that libspdm automatically sizes the request. The three APIs can then be condensed into one

libspdm_return_t libspdm_get_certificate(void *spdm_context,
                                         const uint32_t *session_id,
                                         uint8_t slot_id,
                                         size_t *cert_chain_size,
                                         void *cert_chain,
                                         const void **trust_anchor,
                                         size_t *trust_anchor_size);

The Integrator can set trust_anchor to NULL if they so desire.

[steven-bellock]

Due to #2909 it sounds like length needs to be exposed to the Integrator due to buggy Responders. @jyao1 proposed to keep the current libspdm_get_certificate and just merge libspdm_get_certificate_ex with libspdm_get_certificate_choose_length_ex.