docs: add changelog for message transformation

changes/ee/feat-13199.en.md

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+Implemented the Message Enrichment and Transformation feature.
+
+This allows users to transform incoming messages without the need to define SQL rules in the Rule Engine, just by using simple variform syntax.
+
+For example: if we want to take an incoming message encoded as Avro, decode it to JSON and then take the `tenant` client attribute from the publishing client and prefix it to the topic, before processing the result in Rule Engine, one would only need a transformation with the following configuration:
+
+```hocon
+message_transformation {
+  transformations = [
+    {
+      name = mytransformation
+      enable = true
+      failure_action = drop
+      payload_decoder = {type = avro, schema = myschema}
+      payload_encoder = {type = json}
+      operations = [
+        {key = "topic", value = "concat([client_attrs.tenant, '/', topic])"}
+      ]
+    }
+  ]
+}
+
+```