802.1x: Global and per-interface session limit control

This feature introduces configurable global and per-interface session limits for 802.1X to protect switch resources and ensure predictable scaling. Administrators can define these limits to control the number of concurrent authentication sessions and mitigate the impact of authentication surges. By default, each interface supports up to 100 sessions, with no global session limit enforced. When the configured session limits are exceeded, the switch rejects additional authentication attempts and generates corresponding syslog events, helping maintain system stability and providing visibility into limit violations.