Make NCronJob native AOT ready

[linkdotnet]

Currently, NCronJob is not native AOT ready. That might be a less ideal choice for folks who run this library in the cloud and try to minimize the footprint.

There is a nice article from Microsoft itself: https://devblogs.microsoft.com/dotnet/creating-aot-compatible-libraries/

Basically what we want is:

1. Add the analyzers:

<PropertyGroup>
    <IsAotCompatible>true</IsAotCompatible>
</PropertyGroup>

2. Fix all errors
3. Create a new sample with native aot and publish the app and check if it is still doing its thing.