Support custom (D)TLS signature_algorithms and supported_groups
Support custom (D)TLS signature_algorithms and supported_groups

Enable the users of libcoap to specify (D)TLS signature_algorithms and
supported_groups extension values when used with OpenSSL using optional defines:

  /* COAP_OPENSSL_CIPHERS is already supported by libcoap. */
  #define COAP_OPENSSL_CIPHERS "ECDHE-ECDSA-AES256-GCM-SHA384"
  /* These two defines are added by this patch. Values are case-sensitive. */
  #define COAP_OPENSSL_SIGALGS "ed25519"
  /* Groups are supported only in OpenSSL v1.1.1 and above. */
  #define COAP_OPENSSL_GROUPS "X25519"

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap