* Expose server session items

* Add support for custom session items for injected application messages

* Expose TCP keep alive options to options builder

* Update ReleaseNotes.md

* Expose more options in disconnect options.

* Expose more options in disconnect API.

* Refactor code.

* Expose user properties in disconnected event.

* Expose more disconnect options in server.

* Fix build errors

* Add new samples.

* Fix client cleanup

* Fix merge conflicts

* Add feature validation.

* Update ReleaseNotes.md

* Add validators or subscribe and unsubscribe.

* Add feature validation for disconnect.

* Fix build errors.

* Fix build errors
Co-authored-by: NHansM <HansM2013@gmx.de>

1546 possible bug in server repeatedly sending the same message to client subscribed with cleansession=false (#1569)

1552 mqttnetservermqttclientsessionsmanager subscribersessions is not proper removed on special case (#1562)

fix cross access to async events.