Feat: HEALTH_ADDRESS_TO_PING variable

- Defaults to `1.1.1.1`
- Add more Ping integration tests with different addresses
- Add unit test pinging 127.0.0.1
- Add comment explaining why we need to use ICMP instead of UDP

internal/configuration/settings_test.go

@@ -51,6 +51,7 @@ func Test_Settings_lines(t *testing.T) {
 				"   |--Timezone: NOT SET ⚠️ - it can cause time related issues",
 				"|--Health:",
 				"   |--Server address: ",
+				"   |--Address to ping: ",
 				"   |--VPN:",
 				"      |--Initial duration: 0s",
 				"|--HTTP control server:",