[REQ] spring generator add Nullable annotations

[caleb-cushing0-kr]

Is your feature request related to a problem? Please describe.

Spring provide an @Nullable annotation itself, so this could be used. It'd be nice to generate this along with NotNull for coding assistance and static analysis.

current config

openApiGenerate {
    generatorName = "spring"
    library = "spring-boot"
    inputSpec = "$projectDir/src/main/resources/openapi.json".toString()
    outputDir = layout.buildDirectory.dir("generated/sources/openapi").get().toString()
    library =
        configOptions.putAll([
            openApiNullable        : "false",
            generateApis           : "false",
            generateSupportingFiles: "false",
            serializationLibrary   : "jackson",
            interfaceOnly          : "true",
            annotationLibrary      : "none",
            documentationProvider  : "none",
            useBeanValidation      : "true",
            requestMappingMode     : "none",
            reactive               : "true",
        ])

openapi-generator = { id = "org.openapi.generator", version = "7.0.0" }