OpenAPI: openapi-model-generator-maven-plugin to generate Java types from OpenAPI schemas

[manusa]

Description

Part of #6130

To be able to generate the Java types easily we need a way to invoke the generator project-wise (Maven).
The easiest approach is to create a Maven plugin that can be easily added to each model module and that will be triggered as part of the model build process.

Specifications

• Implement a new Maven module packaged as openapi-model-generator-maven-plugin in kubernetes-model-generator/openapi/maven-plugin:
  io.fabric8:openapi-model-generator-maven-plugin
• This is an internal module, so it shouldn't be published to Maven Central:
  <maven.deploy.skip>true</maven.deploy.skip>
• The plugin should provide support to define the following settings:

  Configuration	Description
  schemas*	List of OpenAPI schema files (*.json) to process
  outputDirectory*	Directory where generated Java classes should be persisted
  packageName*	Name of the base package for the generated Java classes
  builderPackage*	Target package for Sundrio-generated builder classes
  addBuildableReferences	Whether to add buildable references to generated classes (should be true for non-core models)
  packageMappings	Additional mappings to output classes to packages different than packageName
  skipGenerationRegexes	List of definition regexes to skip in the generation
  includeGenerationRegexes	List of definition regexes to include in the generation
  generateJavadoc	Add Javadoc comments from OpenAPI descriptions
  generatedSourcesDirectory	Optional, defaults to $outputDirectory/src/generated/java
  overridesDirectory	Optional, defaults to $outputDirectory/src/main/java

• Generator cleans output directory before processing the schema(s)
• Generator allows for providing multiple schemas
  • Schemas are processed in order
  • The most recent processed schema overwrites any class that has been defined by a previous schema