Extend session index fields mapping with a session creation timestamp

[azasypkin]

Summary

To support initial phase of the current sessions limiting functionality described in #18162 (comment) we need to know when session was created to pick the oldest one to displace.

In the scope of this issue, we need to update Kibana session index mappings to include a new field that would record session creation date (created_at?). Kibana should upgrade the session index mapping (or create a new index and re-index existing sessions) during startup after the upgrade. We also need to update session management service to timestamp newly created sessions.

We also need to decide whether we want to keep old existing session index documents with the empty created_at field or fill it in during upgrade.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)