fix: shutdown and shutdown notification issues [MTT-7791] [MTT-7540]

This PR includes several fixes to the NetworkManager shutdown sequence where:

A client disconnected by a server-host would not receive a notification
A server-host could shutdown during a relay connection but periodically the transport disconnect message to any connected clients could be dropped.
A host could disconnect its local client but remain running as a server
OnClientDisconnectedCallback was not being invoked under certain conditions when disconnected.
OnClientDisconnectedCallback was not returning the assigned client identifier on the client-side when invoked.
If a host or server shutdown while a client owned NetworkObjects (other than the player) it would throw an exception.
If you modify a NetworkVariable or NetworkList during a NetworkManager shutdown an exception was thrown.

fix: #2759 fix: #2705 fix: #2699 fix: #2389 fix: #2064 fix: #1880

Changelog

Fixed: Issue where a client disconnected by a server-host would not receive a local notification.
Fixed: Issue where a server-host could shutdown during a relay connection but periodically the transport disconnect message sent to any connected clients could be dropped.
Fixed: Issue where a host could disconnect its local client but remain running as a server.
Fixed: Issue where OnClientDisconnectedCallback was not being invoked under certain conditions.
Fixed: Issue where OnClientDisconnectedCallback was always returning 0 as the client identifier.
Fixed: Issue where if a host or server shutdown while a client owned NetworkObjects (other than the player) it would throw an exception.
Fixed: Issue where setting values on a NetworkVariable or NetworkList within OnNetworkDespawn during a shutdown sequence would throw an exception.
Changed: The server or host shutdown so it will now perform a "soft shutdown" when NetworkManager.Shutdown is invoked. This will send a disconnect notification to all connected clients and the server-host will wait for all connected clients to disconnect or timeout after a 5 second period before completing the shutdown process.
Changed: OnClientDisconnectedCallback will now return the assigned client identifier on the local client side if the client was approved and assigned one prior to being disconnected.