[default codegen] Correct parent variables being duplicated in child

Created by: mtraynham

Related to #4171 (closed).

When a composed schema has required attributes, it incorrectly adds duplicate optionalVars from the parent which should have been in the requiredVars and deduplicated.

For example:

{
    "Parent": {
        "required": [
            "a"
        ],
        "type": "object",
        "properties": {
            "a": {
                "type": "string"
            }
        }
    },
    "Child": {
        "required": [
            "a",
            "c"
        ],
        "type": "object",
        "allOf": [
            {
                "$ref": "#/components/schemas/Parent"
            },
            {
                "type": "object",
                "properties": {
                    "c": {
                        "type": "string"
                    }
                }
            }
        ]
    }
}

If a template loops required and optional properties separately it generates something like the following, where the property c is duplicated:

data class Child (
    @Json(name = "a")
    val a: kotlin.String,
    @Json(name = "c")
    val c: kotlin.String,
    @Json(name = "c")
    val c: kotlin.String? = null
)

The composedSchema required attributes should be added to both required and allRequired arrays.

I added tests to check both this functionality and where the child has the required property, not the composed model.

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Not sure who to attach to this... @wing328