TypeDescriptor caches prevent AssemblyLoadContext from unloading

[alexey-zakharov]

TypeDescriptor caches prevent AssemblyLoadContext from unloading

ReflectTypeDescriptionProvider captures Type instance in static variable and prevents AssemblyLoadContext that owns the type from unloading.

MetadataUpdateHandler attribute is used to facilitate .NET HotReload and also used in Unity to ensure Type caches are cleared on AssemblyLoadContext unloading.
However ReflectTypeDescriptionProvider currently does not fully clear types that were requested to be removed.
While the cached properties for the type are cleared, the Type itself is kept in the _typeData ConcurrentDictionary.
This prevents AssemblyLoadContext from unloading.

The test at 0a2d601 demonstrates the AssemblyLoadContext leak.

Note: we assume that BCL libraries are safe from AssemblyLoadContext leaks (or can be fundamentally made safe)

[vitek-karas]

This is a duplicate of #30656.