V15: Notification Hub (#17776)

* Initial stab at how this could look

* Authorization PoC wip

* Add connection manager

* Add DI to its own class

* Use enum instead of string

* Use groups

* Refactor group management into its own service

* Update a users groups when it's saved

* Add saved events

* Wire up deleted notifications

* Ensure update date and create date is the same

* Cleanup

* Minor cleanup

* Remove unusued usings

* Move route to constant

* Add docstrings to server event router

* Fix and suppress warnings

* Refactor to authorizer pattern

* Update EventType

* Remove unused enums

* Add trashed events

* Notify current user that they've been updated

* Add broadcast

We don't need it, but seems like a thing that a server event router should be able to do.

* Add ServerEventRouterTests

* Add ServerEventUserManagerTests

* Use TimeProvider

* Remove principal null check

* Don't assign event type

* Minor cleanup

* Rename AuthorizedEventSources

* Change permission for relations

* Exctract event authorization into its own service

* Add some tests

* Update name

* Add forgotten file

* Rmember to add to DI

src/Umbraco.Core/ServerEvents/IServerEventUserManager.cs

@@ -0,0 +1,24 @@
+using System.Security.Claims;
+
+namespace Umbraco.Cms.Core.ServerEvents;
+
+/// <summary>
+/// Manages group access for a user.
+/// </summary>
+public interface IServerEventUserManager
+{
+    /// <summary>
+    /// Adds the connections to the groups that the user has access to.
+    /// </summary>
+    /// <param name="user">The owner of the connection.</param>
+    /// <param name="connectionId">The connection to add to groups.</param>
+    /// <returns></returns>
+    Task AssignToGroupsAsync(ClaimsPrincipal user, string connectionId);
+
+    /// <summary>
+    /// Reauthorize the user and removes all connections held by the user from groups they are no longer allowed to access.
+    /// </summary>
+    /// <param name="user">The user to reauthorize.</param>
+    /// <returns></returns>
+    Task RefreshGroupsAsync(ClaimsPrincipal user);
+}