adjusted csharp codegen to allow for nullable properties

[BryanAldrich]

For value types only unless nullable reference types is enabled.

Edited tests to compensate for the actual data types. They were optional/not-required in the yaml, but came back with non-nullable types.

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[devhl-labs]

Looks good, but embedding the question mark in the data type is something we should try to remove eventually. We can use the nullable property, x-is-value-type, or x-is-nullable-type (true for value or nrt enabled). I've already handled it for generichost.

[BryanAldrich]

So, you're saying to add {{^required}}{{nrt?}}{{^nrt}}{{#vendorExtensions.x-is-value-type}}?{{/vendorExtensions.x-is-value-type}}{{/nrt}} {{/required}} in the mustache template on the properties (i copied that from the generichost template). And then remove the java edit?

[devhl-labs]

Yes, that is one way. We could also add a vendor extension to concat the data type and the ? if we like. This is just a suggestion for a future pr though, you can leave this alone if you want.