Update GroupReport.ID property to base64-encoded String

xxclient/src/main/java/io/elixxir/xxclient/models/GroupReport.kt

 package io.elixxir.xxclient.models
 
 data class GroupReport(
-    val id: ByteArray,
+    val id: String,
     val rounds: List<Long>,
     val roundUrl: String,
     val status: Long