Add mechanism to disable nullable for Kotlin properties

[0xabadea]

The newly-added KotlinNullablePropertyCustomizer results in Kotlin nullable properties being always marked as non-required and nullable. Sometimes that may not be a desirable output.

The Java/Kotlin+Jackson ecosystem does not generally or easily support undefined and null properties at the same time. Usually, Kotlin properties with a null value are either serialized as "property": null or they are left out (making them undefined). In our backend we leave them out via @JsonInclude non_null/non_absent and we document them as non-required in our OpenAPI spec. The understanding is that they may be missing from the payload, but if they are present, they are never null.

Our frontend then generates TypeScript types with optional properties, as in property?: TheType. In this setup, marking these properties as nullable in the OpenAPI spec results in optional and nullable TypeScript properties, as in property?: TheType | null. This brings no additional value and is actually detrimental, as the frontend developers now need to handle null values that never occur in practice.

I propose the introduction of a new property springdoc.model-converters.kotlin-nullable-property-customizer.enabled (defaulted to true) that would allow disabling KotlinNullablePropertyCustomizer on demand.