Allow to trigger a TLS 1.3 KeyUpdate

If you have a long-lived connection that does a lot of data transfer it might help to refresh the session key.

For example during the download or upload of a 4GB file.

This is for the following low-level API https://docs.openssl.org/master/man3/SSL_key_update/

#include <openssl/ssl.h>

int SSL_key_update(SSL *s, int updatetype);
int SSL_get_key_update_type(const SSL *s);

We already have Context.set_timeout which restrict the time after which connections can reuse an already negotiated session ... but if you have a single long lasting session , I am not sure that pyOpenSSL triggers any key updates.

This is only for TLS 1.3

The API is also available for QUIC.. but I don't know if pyOpenSSL supports it.

I am opening this issue here to see if it make sense to expose this API.

Start by reading the existing Context.set_timeout API and the OpenSSL SSL_key_update and SSL_get_key_update_type documentation linked in the issue. Determine how TLS 1.3 key updates would fit pyOpenSSL’s existing connection APIs, including the stated QUIC uncertainty; done means a clearly scoped exposure or a documented decision not to support it.