INT-3730: TCP Expose SSLSession for Header Mapping

JIRA: https://jira.spring.io/browse/INT-3730

Expose the `SSLSession` on `TcpConnection` to support custom header
mapping of properties from the session.

src/reference/asciidoc/ip.adoc

@@ -1359,6 +1359,12 @@ The framework includes acknowledgment information in the data packet.
 | For information only - when using a cached or failover client connection factory, contains the actual underlying connection id.
 |===
 
+For inbound messages, `ip_hostname`, `ip_address`, `ip_tcp_remotePort` and `ip_connectionId` are mapped by the default
+`TcpHeaderMapper`.
+Users can add additional headers by subclassing `TcpHeaderMapper`, overriding the method `supplyCustomHeaders`, and
+providing an instance to the connection factory using the `mapper` property.
+For example, when using SSL, properties of the `SSLSession` can be added by obtaining the session object from the
+`TcpConnection` object which is provided as an argument to the `supplyCustomHeaders` method.
 
 [[ip-annotation]]
 === Annotation-Based Configuration

src/reference/asciidoc/whats-new.adoc

@@ -77,6 +77,15 @@ Also, the `remote-timeout` no longer defaults to the same value as `reply-timeou
 
 See <<tcp-ob-gateway-attributes>> for more information.
 
+[[x4.2-tcp-ssl]]
+==== TCP SSLSession Available for Header Mapping
+
+`TcpConnection` s now support `getSslSession()` to enable users to extract information from the session to add to
+message headers.
+
+See <<ip-msg-headers>> for more information.
+
+
 [[x4.2-inbound-channel-adapter-annotation]]
 ==== @InboundChannelAdapter