Add the GeneratedCodeAttribute to generated code

[rcollette]

Is your feature request related to a problem? Please describe.
Coverlet allows for exclusion of code having specific attributes, such as

dotnet test "My.csproj" -p:ExcludeByAttribute=\"GeneratedCodeAttribute,CompilerGeneratedAttribute\"

But currently this attribute doesn't exist on the generated code, like other source generators such as the .NET Regex source generator.

Describe the solution you'd like
Add GeneratedCodeAttribute to generated classes.

Describe alternatives you've considered
I've tried using file exclusions like:

dotnet test "My.csproj" -p:ExcludeByAttribute=\"GeneratedCodeAttribute,CompilerGeneratedAttribute\" -p:ExcludeByFile=\"**/*.g.cs\"

But mapperly generated code still shows up in coverage reports.

Along with messages the the related .g.cs file no longer exists.

[latonz]

Feel free to open a PR implementing this. Some starting points:

• Contributors docs
• Riok.Mapperly.Emit.SourceEmitter

[github-actions]

🎉 This issue has been resolved in version 3.4.0-next.3 🎉

The release is available on:

• GitHub release
• v3.4.0-next.3

Your semantic-release bot 📦🚀

[github-actions]

🎉 This issue has been resolved in version 3.4.0 🎉

The release is available on:

• GitHub release
• v3.4.0

Your semantic-release bot 📦🚀