[JavaSpring][21200] improve Kotlin interopability with optional values

[ondrej-simon]

nullable annotations are now added to getters, setters, parameters, for optional attributes without default value

Fixes #21200

For objects, Kotlin interopability works by inferring nullability annotations from getters and setters. Having Nullable annotation only on a member variable is not enough for Kotlin to know a property is nullable. This PR improves on this.

This PR also fixes problem where optional parameters without default values (e.g. headers, query parameters, cookies, request body) are not marked with nullable annotation, acting as if they were always present even when it is not the case.

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[ondrej-simon]

The parameter schemaMappings was not used at all by this method and was:

• either populated with variable importMappings (mismatch between parameter name and actual argument),
• or an empty map,

so I decided to clean up the duplicated method and keep only a single one.

[wing328]

i suggest doing it in a separate pr instead as changes in default codegen requires a lot more time and effort to review

[ondrej-simon]

No problem, Will, fair point. Changes reverted. Thank you for the hint.

[ondrej-simon]

The pathParams.mustache file is intentionally left unaffected, because path params must always be required as per OAS and thus should/must never be null.