Fix NtpTimeProvider

`timeInfo.getReturnTime()` is the _local_ timestamp of the ntp reply.

To properly use the Ntp it is needed to:
- call `timeInfo.computeDetails()` to compute the NTP offset
- Add `timeInfo.getOffset()` to the local timestamp

Author: paulo-raca

totp/src/main/java/dev/samstevens/totp/exceptions/TimeProviderException.java

@@ -1,6 +1,9 @@
 package dev.samstevens.totp.exceptions;
 
 public class TimeProviderException extends RuntimeException {
+    public TimeProviderException(String message) {
+        super(message);
+    }
     public TimeProviderException(String message, Throwable cause) {
         super(message, cause);
     }

totp/src/main/java/dev/samstevens/totp/time/NtpTimeProvider.java

@@ -37,13 +37,19 @@ private NtpTimeProvider(String ntpHostname, int timeout, String dependentClass)
 
     @Override
     public long getTime() throws TimeProviderException {
+        TimeInfo timeInfo;
         try {
-            TimeInfo timeInfo = client.getTime(ntpHost);
-
-            return (long) Math.floor(timeInfo.getReturnTime() / 1000L);
+            timeInfo = client.getTime(ntpHost);
+            timeInfo.computeDetails();
         } catch (Exception e) {
             throw new TimeProviderException("Failed to provide time from NTP server. See nested exception.", e);
         }
+
+        if (timeInfo.getOffset() == null) {
+            throw new TimeProviderException("Failed to calculate NTP offset", null);
+        }
+
+        return (System.currentTimeMillis() + timeInfo.getOffset()) / 1000;
     }
 
     private void checkHasDependency(String dependentClass) {