Add OpenAPI Normalizer

Add OpenAPI Normalizer to normalize the spec before further processed by OpenAPI Generator.

Only 1 rule is supported to start with: REF_AS_PARENT_IN_ALLOF. When it's set to true, the child schema of a allOf schema will be considered a parent of the object if it's a $ref.

This is done by adding x-parent: true to the parent schema to indicate it's a parent without the use of discriminator. (x-parent can also be used to set the type of the parent schema, e.g. x-parent: abstract to indicate a abstract base class)

cc @OpenAPITools/generator-core-team

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.