feat: add TLS certificate expiry check to HTTPS network monitors (#2401)

Co-authored-by: henrygd <hank@henrygd.me>

internal/entities/monitor/monitor.go

@@ -27,6 +27,13 @@ type Config struct {
 	Interval uint16 `cbor:"4,keyasint"` // seconds
 }
 
+// CertInfo holds details of the leaf TLS certificate presented by a target.
+type CertInfo struct {
+	// Expires is the certificate's NotAfter Unix timestamp in milliseconds.
+	Expires int64  `cbor:"0,keyasint" json:"expires"`
+	Issuer  string `cbor:"1,keyasint,omitempty" json:"issuer,omitempty"`
+}
+
 // SyncRequest defines an incremental or full monitor sync request sent to the agent.
 type SyncRequest struct {
 	Action  SyncAction `cbor:"0,keyasint"`
@@ -76,6 +83,8 @@ type Result struct {
 	TotalCount   int64 `cbor:"10,keyasint"`
 	SuccessCount int64 `cbor:"11,keyasint"`
 	ResponseSum  int64 `cbor:"12,keyasint"`
+	// Cert is set for HTTPS targets when a certificate check has new info the hub has not stored yet.
+	Cert *CertInfo `cbor:"13,keyasint,omitempty"`
 }
 
 // Stats holds response times in microseconds and packet loss percentage (0-100).