Returns an object that can derive key material from the agreed connection secrets.
See [RFC 5705][] for more details on what this is for.
This function can be called at most once per connection.
This function will error:
- if called prior to the handshake completing; (check with
[`CommonState::is_handshaking`][crate::CommonState::is_handshaking] first).
- if called more than once per connection.
[RFC 5705]: https://datatracker.ietf.org/doc/html/rfc5705