fix: omitting optional property fails C# compilation

[rix0rrr]

The following is a minimal reproducing example:

export interface ISomeInterface {
  readonly xyz?: string;
}

export abstract class SomeClass implements ISomeInterface {
}

This breaks code generation for C#, as the SomeClass._Proxy class we generate will try to generate an override for the Xyz property, which is missing from the SomeClass class itself.

This issue is exhibited by a difference in logic between how we iterate the members of a class when we generate the class itself vs when we generate its proxy.

However, after looking at the code I'm not convinced it's correct either way (nor for Java), so to be safe we picked the solution where we force the user to declare the property on the class.

The correct declaration would be:

export abstract class SomeClass implements ISomeInterface {
  public abstract readonly xyz?: string;
}

Related: https://github.com/aws/aws-cdk/pull/31946/files

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[aws-cdk-automation]

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Status	Branch	Result
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✅	maintenance/v5.4

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