Add offset, retryCount, & sessionDuration to OptionalHeaders

Motivation: support passing values as headers, add comments

Author: Rodrigo Gomez Palacio

OneSignalSDK/onesignal/core/src/main/java/com/onesignal/core/internal/http/impl/OptionalHeaders.kt

@@ -1,5 +1,20 @@
 package com.onesignal.core.internal.http.impl
 
 data class OptionalHeaders(
+    /**
+     * Used as an E-Tag
+     */
     val cacheKey: String? = null,
+    /**
+     * Used for read your write consistency
+     */
+    val rywToken: String? = null,
+    /**
+     * Current retry count
+     */
+    val retryCount: Int? = null,
+    /**
+     * Used to track delay between session start and request
+     */
+    val sessionDuration: Long? = null,
 )