[default codegen] Correct parent variables being duplicated in child

[mtraynham]

Related to #4171.

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 classChild (
@Json(name ="a")
vala: kotlin.String,
@Json(name ="c")
valc: kotlin.String,
@Json(name ="c")
valc: 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.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

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

[mtraynham]

@wing328 any feedback on this would be appreciated!

[mtraynham]

@jimschubert, you may also have some feedback here.

[wing328]

@mtraynham thanks for the PR, which looks good to me.

[wing328]

@mtraynham thanks for the PR, which has been included in the v4.2.2 release: https://twitter.com/oas_generator/status/1201432648544972800