These functions are similar to SSL_CTX_set_cookie_{generate,verify}_cb,
but used for the application-controlled portion of TLS1.3 stateless
handshake cookies rather than entire DTLSv1 cookies.
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5463)