feat(openapi/generator): add the x-omitempty extension

[karimassaly]

Contexte to this upgrade

We currently do not have the possibility to decide weither a value should omit empty or not once the openAPI format is transcribed to Golang structures.

I have added a new value to the schema and reference structures called Extensions. This value will be able to handle all extra extensions we would like our components to hold. As of now only the "x-omitempty" is handled as it will search the json tag in a structure and place the value of this extension either to true or false.

But many other extensions could be added later on.

[tbourrelovh]

Hi,
Let me add more context.
We use fizz to generate our API specifications, and then rely on go-swagger to generate client code so that our customers can use it to call the API.

We observed that go-swagger adds the following struct tag on fields by default : json:"omitempty".
An issue on go-swagger states that to override this behavior, the x-omitempty openapi extension must be set.

We want to be able to have x-omitempty extension supported in order to have a generated client aligned with the api and avoid misleading our users.

[wI2L]

I don't get the rationale for this field addition.
Extensions should only be present on the Schema struct. It can then be used inline or be references through the spec components.

[wI2L]

If I get it right, you're trying to simulate the new omitnil JSON field tag, to avoid marshaling the Extensions field it it's empty ? What the purpose.
omitnil could be used (from go1.24), and for previous versions, it doesn't really matter if we marshal extensions: [].

[wI2L]

@karimassaly

With your current implementation, the new extension is always enabled.
We currently use OperationOption in order to configure each operation individually.

For this usecase, I envision the addition of a new GeneratorOption type that would allow to configure the behavior of the generator when it's initialized with NewGenerator. The pattern would allow to pass a variadic list of options to New and NewFromEngine, which would be backward compatible.

The option func could be named WithExtension, to be generic, and it would itself accept a variadic list of extensions names (from constants). Store the content in a map (for O(1) lookup) or a new generatorExtensions struct with boolean fields if that's more convenient, and use the condition where that's necessary to decide if the extensions should be enabled (in the addExtensions for example).