Expose TimeZoneChangedEvent in Maui.Essentials

[Mrnikbobjeff]

Description

Detecting when a running Maui app crosses into a new timezone is too complicated. I propose an cross-platform Essential that invokes an event (much like the Connectivity api) that we can handle.
In order to facilitate proper date handling, a new event is added to the Essentials platform that fires whenever the timezone changes on any platform.

A consumer would only see the new event that can be implemented. The handler would give us a place to call TimeZoneInfo.ClearCachedData()

This would close the gap exposed by https://docs.microsoft.com/en-us/dotnet/api/system.timezone.currenttimezone?view=netstandard-2.1#notes-to-callers

"If the system's local time zone subsequently changes, the CurrentTimeZone property does not reflect this change. If you need to handle time zone changes while your application is running, use the TimeZoneInfo class and call its ClearCachedData() method."
(Duplicate issue from Xamarin.Essentials after end of support)

Public API Changes

	{
		public static event EventHandler<TimeZoneChangedEvent> TimezoneChanged
		{
			add => Default.TimezoneChanged+= value;
			remove => Default.TimezoneChanged-= value;
		}
		public static bool IsSupported => Default.IsSupported;

		public static bool IsMonitoring
			=> Default.IsMonitoring;

		public static void Start(SensorSpeed sensorSpeed)
			=> Default.Start(sensorSpeed);

		public static void Stop()
			=> Default.Stop();

		static ITimezoneTracker? defaultImplementation;
		public static ITimezoneTracker Default =>
			defaultImplementation ??= new TimezoneTrackerImplementation();

		internal static void SetDefault(ITimezoneTracker? implementation) =>
			defaultImplementation = implementation;
	}```

### Intended Use-Case

Apps which need to continue working with dates if the user switches timezones.