Support Optional Elements

### Motivation

Currently, all content element properties are generated like they are required, i.e. that they will always have a value, yet kontent.ai allows you to mark whether a content element is required or not. Non required properties should be optional in the model.

### Proposed solution

`myElement: Type` for required elements and `myElement?: Type` for optional elements.

### Additional Comments

This is likely a breaking change for consumers, especially those running strict Typescript because they will now have to write code to handle checking whether optional properties have a value or not.

This could potentially be offset by adding a CLI flag for supporting optional elements so this feature is opt in.