Builds a linked list of x509_crt instances, all of which are owned by mbedtls. That is, the
memory for these certificates has been allocated by mbedtls, on the C heap. This is needed for
situations in which an mbedtls function takes ownership of a list of certs. The problem with
handing such functions a “normal” cert list such as certificate::LinkedCertificate or
certificate::List, is that those lists (at least partly) consist of memory allocated on the
rust-side and hence cannot be freed on the c-side.