Read platform options from runtime config

[maxkatz6]

Is your feature request related to a problem? Please describe.

Currently, any platform option, such as Win32PlatformOptions { RenderingMode = {} } is defined only via C# code.
There is a number of cases, when runtime config, defined on the project level, is move convenient and flexible approach. This is also a common approach for .NET runtime features.

In perspective, if we migrate completely to runtime configs, and drop C# classes, we could support trimming of unused code compile time as well.

Describe the solution you'd like

It's up to debate how we should prioritize runtime config options compared to C# platform options.
But I would suggest:

1. If runtime option is defined, with either value - use it. Completely ignore C# platform options, allowing as to do trimming in perspective.
2. If runtime option is not defined, read C# platform options. It also helps with the fact, that most of our platform options are not nullable, and always have at least some value.
3. If none is defined, fallback to what Avalonia considers default behavior in current release.

Describe alternatives you've considered

No response

Additional context

No response