[Feature][Csharp][GenericHost] Option to be able to use object initializers for model instantiation

[Irame]

This PR adds useObjectInitializerApproach to additional-properties for the csharp generator when using the generichost library. With this flag enabled the properties of generated model classes have the required keyword if they are required and use an init setter if they are read-only. It also generates a parameter less constructor if there are required properties in the model to allow the use of the object initializer and adds the SetsRequiredMembers attribute to the other constructors.

This should fix the issue #17147 at least when using the generichost library.

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@mandrean, @shibayan, @Blackclaws, @lucamazzanti, @iBicha

[devhl-labs]

Does this have to be behind the new useObjectInitialzerApproach switch? I think you can use net80OrLater instead.

Why is it also behind hasRequired?

[Irame]

I put it behind the hasRequired switch so it is only generated when the normal constructor does have some mandatory arguments to prevent ambiguity.
And I put it behind the useObjectInitializerApproach switch to not generate this constructor without also adding the required keyword to properties. This is necessary to prevent the instantiation of the model without specifying the required fields.

Yes, the useObjectInitializerApproach could be replaced by the net80OrLater or even the net70OrLater switch, but I didn't want to introduce any changes to currently generated code. Should I change it?

[devhl-labs]

I haven't used the feature myself, but I don't see any reason to not use net70OrLater since it appears to just be a syntax change.

[Irame]

Yes its basically a syntax change and the old syntax can still be used.
The only thing I see that is different using the new approach is, that OnCreated is not called. I didn't include it in the parameter less constructor, because it would get called before the required properties are set in contrast to the current constructor where it gets called after all properties are set.

[Irame]

Ok, so based on the review I will remove the useObjectInitializerApproach option completely and replace it with net70OrLater.

[wing328]

@Irame can you please resolve the merge conflicts when you've time?

if no further feedback from anyone, I'll merge it before the upcoming 7.3.0 release.

cc @devhl-labs