provide API to track resumed sessions

Traditionally, `SSL_get_session` and `SSL_session_get_id` were used to track connections that were being resumed using the secrets derived from a particular full handshake.

We need to provide similar API at server side for debugging network issues related to resumption (e.g. buggy intermediaries that tries to do clever things).

No file, test, or entry point is named. Start by locating the server-side session-resumption handling and existing session-tracking interfaces, then clarify with maintainers what API data and debugging behavior are required.

Written by the indexing model from the issue text.