Make the model classes partial in C#

[vvdb-architecture]

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

In other code generators (did anyone say NSwag?) , the generated model classes are partial. This can be very handy to add some computed properties as part of the API, without forcing users to define wrapper classes.
It may not be applicable to other languages not having the partial class concept, but it surely would help those who use C#.

Client library/SDK language

Csharp

Describe the solution you'd like

generate public partial class instead of public class when generating model code.

Additional context

No response

[baywet]

Hi @vvdb-architecture
Thanks for using kiota and for reaching out.
We've had multiple requests about that #4242 among others.
Although my personal opinion is that we shouldn't do this (unique to CSharp, does not provide much more value than extension methods), I think this is evidence the community wants it.

This gets written here

kiota/src/Kiota.Builder/Writers/CSharp/CodeClassDeclarationWriter.cs

Line 41 in 0254c19

writer.WriteLine($"public class {codeElement.Name.ToFirstCharacterUpperCase()} {derivation}");

And tested there

kiota/tests/Kiota.Builder.Tests/Writers/CSharp/CodeClassDeclarationWriterTests.cs

Line 54 in 0254c19

Assert.Contains("public class", result);

Is this something you'd be willing to submit a pull request for?

[vvdb-architecture]

Here you go @baywet: #4895.

That should be much easier than correcting #4893 which is just... strange.