[backport 7.9] For CMS Preview and PageComposer use a dedicated CmsSessionContext object to share state with the HST web application(s)

This concerrns the HST side of the implementation of CMS-10424.

In addition to using the new CmsContextService and CmsSessionContext to coordinate with the CMS session state,
this will also deprecate the usage of the following org.hippoecm.hst.core.container.ContainerConstants:

• CMS_SSO_REPO_CREDS_ATTR_NAME
• (Boolean) CMS_SSO_AUTHENTICATED
• CMS_USER_ID_ATTR

These session attributes are replaced by the new request attributes (set upon each authorized CMS request):

• CMS_REQUEST_REPO_CREDS_ATTR
• CMS_REQUEST_USER_ID_ATTR

To determine if a HST HttpSession is authorized to process a CMS request, instead of checking the CMS_SSO_AUTHENTICATED session attribute, use:

CmsRequestContext.getContext(HttpSession) != null

The deprecated session attributes are still set to retrain backwards compatibility but no longer used internally.