* 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.

commit 469caf65
Author: Christian <6939810+chkr1011@users.noreply.github.com>
Date:   Sun Jul 31 10:55:40 2022 +0200

    Fix build errors.

commit 42402fba
Author: Christian <6939810+chkr1011@users.noreply.github.com>
Date:   Sun Jul 31 10:22:23 2022 +0200

    Allow interception of injected application messages.

commit a0dc8ab0
Author: William Lee <31092948+william19821101@users.noreply.github.com>
Date:   Sun Jul 31 15:50:14 2022 +0800

    Update Server_Simple_Samples.cs (#1474)

    Change e.UserName to e.Username