Support Hot Reload in .NET 6

[davidwengier]

Add a MetadataUpdateHandler attribute, grab all IReloadable instances from DI, and call the Reload method.

Things that need to be IReloadable:

• Screen caches
• Image caches
• Track renderers that cache paths

See https://github.com/dotnet/runtime/pull/51466 for how to do it.

I did it and its easy enough, but the SDK isn't out yet so haven't checked anything in.