allow capture to override timestamp with datetime or datetimeoffset

Author: darkopia

src/PostHog/PostHogClient.cs

@@ -131,11 +131,17 @@ public bool Capture(
         GroupCollection? groups,
         bool sendFeatureFlags)
     {
+        // Check for custom timestamp in properties
+        var timestamp = properties?.TryGetValue("timestamp", out var timestampValue) == true && 
+            (timestampValue is DateTimeOffset customTimestampOffset || timestampValue is DateTime customTimestamp)
+            ? timestampValue is DateTimeOffset dtOffset ? dtOffset : new DateTimeOffset((DateTime)timestampValue)
+            : _timeProvider.GetUtcNow();
+
         var capturedEvent = new CapturedEvent(
             eventName,
             distinctId,
             properties,
-            timestamp: _timeProvider.GetUtcNow());
+            timestamp: timestamp);
 
         if (groups is { Count: > 0 })
         {