Per CommanderStorm's feedback, instead of creating a separate TLS monitor
type, add the TLS alert checking functionality directly to the existing
TCP monitor.
Changes:
- Add TLS_ALERT_CODES, parseTlsAlertNumber(), getTlsAlertName() to tcp.js
- Add checkTlsAlert() method to TCPMonitorType for mTLS verification
- Add 'Expected TLS Alert' dropdown to TCP monitor UI
- Remove separate TLS monitor type (tls.js)
This allows users to verify mTLS endpoints reject connections without
client certificates by expecting specific TLS alerts like
'certificate_required'.
Closes#5837