docs: add changelog for PR #12750

changes/ce/feat-12750.en.md

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+Customizable client attributes in `clientinfo`.
+
+Introduced a new field `client_attrs` in the `clientinfo` object.
+This enhancement enables the initialization of `client_attrs` with specific
+attributes derived from the `clientinfo` fields upon accepting an MQTT connection.
+
+### Initialization of `client_attrs`
+
+- The `client_attrs` field can be initially populated based on the configuration from one of the
+  following sources:
+  - `clientid`: The MQTT client ID provided by the client.
+  - `username`: The username provided by the client.
+  - `cn`: The common name from the TLS client's certificate.
+  - `dn`: The distinguish name from the TLS client's certificate, i.e., the certificate "Subject".
+
+### Extension through Authentication Responses
+
+- Additional attributes may be merged into `client_attrs` from authentication responses. Supported
+  authentication backends include:
+  - **HTTP**: Attributes can be included in the JSON object of the HTTP response body through a
+    `client_attrs` field.
+  - **JWT**: Attributes can be included via a `client_attrs` claim within the JWT.
+
+### Usage in Authorization
+
+- The `client_attrs` can be utilized in authorization configurations or request templates, enhancing
+  flexibility and control. Examples include:
+  - In `acl.conf`, use `{allow, all, all, ["${client_attrs.namespace}/#"]}` to apply permissions
+    based on the `namespace` attribute.
+  - In other authorization backends, `${client_attrs.namespace}` can be used within request templates
+    to dynamically include client attributes.