Sessions were already a core mechanism, but there was no API for them.
There is now a new Session interface that is available via the Request API.  It is intended only for users of sessions.
The previous concrete class Session has been renamed to ManagedSession and it is used by classes that extend AbstractSessionManager.

…core-sessions

# Conflicts:
#	jetty-core/jetty-session/src/main/java/org/eclipse/jetty/session/SessionManager.java
#	jetty-core/jetty-session/src/test/java/org/eclipse/jetty/session/AbstractSessionManagerTest.java
#	jetty-core/jetty-session/src/test/java/org/eclipse/jetty/session/SimpleSessionHandler.java
#	jetty-core/jetty-session/src/test/java/org/eclipse/jetty/session/TestableSessionManager.java
#	jetty-ee10/jetty-ee10-servlet/src/main/java/org/eclipse/jetty/ee10/servlet/ServletApiRequest.java
#	jetty-ee10/jetty-ee10-servlet/src/main/java/org/eclipse/jetty/ee10/servlet/ServletContextResponse.java
#	jetty-ee10/jetty-ee10-servlet/src/main/java/org/eclipse/jetty/ee10/servlet/SessionHandler.java
#	jetty-ee10/jetty-ee10-servlet/src/main/java/org/eclipse/jetty/ee10/servlet/security/authentication/LoginAuthenticator.java
#	jetty-ee9/jetty-ee9-nested/src/main/java/org/eclipse/jetty/ee9/nested/Request.java
#	jetty-ee9/jetty-ee9-nested/src/main/java/org/eclipse/jetty/ee9/nested/Response.java
#	jetty-ee9/jetty-ee9-nested/src/main/java/org/eclipse/jetty/ee9/nested/SessionHandler.java
#	jetty-ee9/jetty-ee9-security/src/main/java/org/eclipse/jetty/ee9/security/authentication/LoginAuthenticator.java

Improved javadoc
used util Attributes interface.