Observability for TLS session resumption

[xnuter]

Problem:
Currently there are not many ways to get insights into session resumption metrics.
Such as:

1. If resumption was attempted. Currently it can be obtained it for session ids via s2n_config_set_cache_retrieve_callback (which is called when session id is provided). However, no way to check if a ticket was provided.
2. Successful resumption. Can be determined via s2n_connection_is_session_resumed.
3. Session age both resumed and expired. So it would be possible to build a histogram of session ages and tune key lifetime and rotation settings.
4. Failed resumptions by category:

• Expired session.
• Expired session ticket.
• Missing key.
• Unknown session ticket.
• Other (if any).

Proposed Solution:

Add methods for accessing the missing items.