Display a badge for messages decrypted using shared keys (#6023)

The EXA side of element-hq/element-meta#2877: if the keys for a message have been forwarded by another user, indicate that in the UI via the text shown when tapping the event shield.

libraries/matrix/test/src/main/kotlin/io/element/android/libraries/matrix/test/timeline/TimelineFixture.kt

@@ -77,6 +77,8 @@ fun anEventTimelineItem(
     timelineItemDebugInfoProvider = debugInfoProvider,
     messageShieldProvider = messageShieldProvider,
     sendHandleProvider = sendHandleProvider,
+    forwarder = null,
+    forwarderProfile = null,
 )
 
 fun aProfileDetails(