[BUG] List of missing or broken APIs moving from swagger-codegen to openapi-generator

Bug Report Checklist

• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?

Description

I'm transitioning from swagger-codegen to openapi-generator and some errors are being thrown after updating my gradle.build file.

• Subclasses of DefaultCodegen(), CodegenConfig can't override
  • preprocessSwagger(swagger:)
  • getSwaggerType(property:)
  • getTypeDeclaration(property:)
  • postProcessOperations(objs:)
  • addAdditionPropertiesToCodeGenModel(codegenModel:, swaggerModel:)
  • fromModel(name: String, model: Model, allDefinitions: MutableMap<String, Model>?): CodegenModel
  • fromOperation(path: String?, httpMethod: String?, operation: Operation?, definitions: MutableMap<String, Model>?, swagger: Swagger?): CodegenOperation
  • no versions of getTypeDeclaration() accepts Property types

because those functions don't exist on supertypes.

• Missing or private properties on CodegenConfigurator

val configurator = CodegenConfigurator()
configurator.lang = lang // missing
configurator.inputSpec = parsed['i'] // private
configurator.outputDir = parsed['o'] // missing

openapi-generator version

4.2.1

Start by reproducing the Gradle-based migration with openapi-generator 4.2.1, then inspect DefaultCodegen, CodegenConfig, and CodegenConfigurator for the listed missing overrides and properties. Compare the affected APIs with the reported swagger-codegen usage; done means each incompatibility is verified and either addressed or clearly documented.