[BUG][jaxrs-spec] openApiNullable doesn't appear to be respected

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The openApiNullable option on the jaxrs-spec generator doesn't appear to be used. It would be nice to have this functionality since I want to have request objects which have nullable fields so that I can know whether or not a field is being set to null or if it simply wasn't provided in the payload if it's optional. I see that other Java generators seem to support this option so it would be nice to have.

openapi-generator version

5.0.1

OpenAPI declaration file content or url

N/A. I didn't think it was necessary to add the full spec, but I can if needed.

Generation Details

Using the Maven plugin with the following configuration:

                <configuration>
                    <inputSpec>${project.basedir}/src/main/resources/openapi.yaml</inputSpec>
                    <output>${project.build.directory}/generated-sources/openapi</output>
                    <apiPackage>com.foo.bar.web.api.v1</apiPackage>
                    <modelPackage>com.foo.bar.web.api.v1</modelPackage>
                    <generatorName>jaxrs-spec</generatorName>
                    <configOptions>
                        <interfaceOnly>true</interfaceOnly>
                        <generateBuilders>false</generateBuilders>
                        <useSwaggerAnnotations>false</useSwaggerAnnotations>
                        <openApiNullable>true</openApiNullable>
                        <generatePom>false</generatePom>
                    </configOptions>
                </configuration>

Steps to reproduce

1. Create a schema which has at least 1 nullable property.
2. Run the openapi generator maven plugin with the provided configuration.
3. The resulting POJO's do not use JsonNullable

Suggest a fix

I'm a new user so I don't fully understand how this works, but it looks like this is probably just an update to some templates? I see the openApiNullable option referenced in other generators such as the one for Java clients.

Start with the jaxrs-spec generator configuration and its model-generation templates, then compare how the other Java generators reference openApiNullable and JsonNullable. Reproduce with the Maven plugin configuration and a schema containing a nullable property; done means the generated jaxrs-spec POJO uses JsonNullable for that property.