[csharp] Handling of nullable and required properties

[Bouillie]

• Non-required field are now wrapped in an "Option" struct (as seen in the generichost generator)
• Nullable fields use the nullable operator "?"
• Fixes invalid model validation on required nullable fields (null values would fail even though they're valid)

The update concerns the restsharp, httpclient and unitywebrequest libraries.

Example of generated code:
public int RequiredNotnullableintegerProp { get; set; }
public int? RequiredNullableIntegerProp { get; set; }
public Option<int?> NotRequiredNullableIntegerProp { get; set; }
public Option<int> NotRequiredNotnullableintegerProp { get; set; }

Those are breaking changes because of the type change of fields and function parameters from T? to Option<T>

Notes : I made sure not to impact the generichost library as it already handles Options. I also took null reference types into account.

Fixes: #22254 #21090

@mandrean @shibayan @Blackclaws @lucamazzanti @iBicha

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[Bouillie]

I've checked the failing tests.

• 4 tests are failing due to github changes that are not related to my changes.
• 1 test fails because I made changes to PetApiTests to reflect the breaking changes of my update, they're supposed to be made. What should I do ?