Add option to generate ExcludeFromCodeCoverage on source generated objects

[zachcwillson]

Is your feature request related to a problem? Please describe.
Depending on team tooling, adding excludes for each class or namespace that gets added can be cumbersome.

Describe the solution you'd like
A new bool value added to the NativeMethods.txt schema, maybe named excludeFromCodeCoverage, that when set to true (default false), will add [ExcludeFromCodeCoverage(Justification = "Source Generated Object")] to all generated classes.

Describe alternatives you've considered

• Add partial class/struct entries in our project for each type and adding the attribute.
• Configuration of code coverage settings file

Additional context
N/A

[AArnott]

I wonder how many people actually want this code included in code coverage.
Maybe we should just always apply the attributes, without adding an option.