[csharp] Mark optional fields in generated client properties as nullable

[ryanisaacg]

Resolves #4816

Mark all optional properties of in the generated C# client as nullable, with the ?marker. For reference types, this enables C# 8's nullable reference types feature which is a nice bonus.

For value types, this fixes an issue with optional values. By default, an optional integer will be expressed as an int in C# with EmitDefaultValue = false. This means that a value of 0 is used to indicate the property is absent, which prevents actually indicating "set this value to 0". This PR emits an int?, which uses null to indicate absence. 0 can now be properly expressed.

This change seems like a breaking change, unless it's put behind some kind of generator flag. I'm perfectly happy to add a generator flag or try to retarget this PR to the 8.0.x branch, whichever would be better.

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@mandrean @shibayan @Blackclaws @lucamazzanti @iBicha

[devhl-labs]

This is an improvement, so I don't object, but I think it is a work around. The better fix is to use wrap not required properties and parameters in a struct to track whether or not it was set ie var foo = new Optional<int?>(); This allows you to explicitly set things to 0 and null. If the value was never set (as tracked by the Optional struct), then serializing should not add the property to the output. I already implemented this in generichost #16810 but it is not a small change.

[wing328]

I've filed #18258 to set primitive types to nullable. If that works for the use cases described, then we can skip this PR.

[ryanisaacg]

I'm no longer working on the project that prompted this PR and an alternative was merged, so I'll close it.

[wing328]

@ryanisaacg that's ok. Thanks again for the PR. All the best to your projects.