feat: add user_tailnet_connections to telemetry (#17018)

## Summary
- Add UserTailnetConnection struct to track desktop client connections
- Add new field to Snapshot struct for telemetry
- Data collection to be implemented in a future PR

relates to coder/nexus#197

coderd/telemetry/telemetry.go

@@ -1149,6 +1149,7 @@ type Snapshot struct {
 	NetworkEvents                        []NetworkEvent                        `json:"network_events"`
 	Organizations                        []Organization                        `json:"organizations"`
 	TelemetryItems                       []TelemetryItem                       `json:"telemetry_items"`
+	UserTailnetConnections               []UserTailnetConnection               `json:"user_tailnet_connections"`
 }
 
 // Deployment contains information about the host running Coder.
@@ -1711,6 +1712,16 @@ type TelemetryItem struct {
 	UpdatedAt time.Time `json:"updated_at"`
 }
 
+type UserTailnetConnection struct {
+	ConnectedAt         time.Time  `json:"connected_at"`
+	DisconnectedAt      *time.Time `json:"disconnected_at"`
+	UserID              string     `json:"user_id"`
+	PeerID              string     `json:"peer_id"`
+	DeviceID            *string    `json:"device_id"`
+	DeviceOS            *string    `json:"device_os"`
+	CoderDesktopVersion *string    `json:"coder_desktop_version"`
+}
+
 type noopReporter struct{}
 
 func (*noopReporter) Report(_ *Snapshot)            {}