Annotate with nullable attributes

Is your feature request related to a problem? Please describe.
Currently the nullable feature is not enabled on most projects. Instead we have some old custom attributes (such as [NoNull]) that are implemented in Stride.Core. These attributes are compatible with Resharper but are ignored by the actual .NET analyzers.

Describe the solution you'd like
Annotate our code with attributes from https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis. Then gradually enable the nullable feature on projects.

Describe alternatives you've considered
None. It can be really useful to detect issues and is expected in modern codebase.

Additional context
It will likely result in lots of warnings. To reduce overload, we can start with the editor and assets assemblies and then the core assemblies. Later apply to the remaining engine assemblies.

Start by reviewing the existing custom attributes in Stride.Core and the editor and assets assemblies identified as the first targets. Compare them with the System.Diagnostics.CodeAnalysis attributes, then assess the warnings produced when nullable analysis is enabled. Done means the selected assemblies are annotated and nullable warnings are reduced before expanding to the core and remaining engine assemblies.