feat(healthcheck): HEALTH_ICMP_TARGET_IP -> HEALTH_ICMP_TARGET_IPS

- Specify fallback ICMP IP addresses
- Defaults changed from 1.1.1.1 to 1.1.1.1,8.8.8.8
- Small periodic check cycles through addresses as it fails and moves to retry

internal/configuration/settings/settings_test.go

@@ -60,7 +60,9 @@ func Test_Settings_String(t *testing.T) {
 |   ├── Target addresses:
 |   |   ├── cloudflare.com:443
 |   |   └── github.com:443
-|   ├── ICMP target IP: 1.1.1.1
+|   ├── ICMP target IPs:
+|   |   ├── 1.1.1.1
+|   |   └── 8.8.8.8
 |   └── Restart VPN on healthcheck failure: yes
 ├── Shadowsocks server settings:
 |   └── Enabled: no