Microsoft.Data.SqlClient is not compatible with AssemblyLoadContext unloading

See https://github.com/dotnet/runtime/issues/31377 for context. The fundamental issue is that unloadability is based on GC references, so SqlClient containing a strong references via event handlers and similar. The suggested fix is that for callbacks like the TimerCallback in Microsoft.Data.SqlClient.SqlConnectionFactory, support for listening to the Unload event could be added and these roots could be cleared, so that the app could be unloaded.