This adds a minimal, callback-based API to import external PSK,
following RFC 9258.  The client and the server importing external PSK
are supposed to set a callback to retrieve PSK, which returns flags
that may indicate the PSK is imported, along with the key:

  typedef int gnutls_psk_client_credentials_function3(
          gnutls_session_t session,
          gnutls_datum_t *username, gnutls_datum_t *key,
          gnutls_psk_key_flags *flags);

  typedef int gnutls_psk_server_credentials_function3(
          gnutls_session_t session,
          const gnutls_datum_t *username, gnutls_datum_t *key,
          gnutls_psk_key_flags *flags);

Those callbacks are responsible to call
gnutls_psk_format_imported_identity() for external PSKs to build a
serialized PSK identity, and set GNUTLS_PSK_KEY_EXT in flags if the
identity is an imported one.

Signed-off-by: Daiki Ueno <ueno@gnu.org>