[java-micronaut] Allow default implementation to return HTTP 501

Created by: auke-

The default controller implementation returns an empty response. While this might be an expected default for some users we want the default implementation to return HTTP 501 Not Implemented instead. This will support rapid development when the specs aren't finalized yet and thus change often while preventing the methods from being unimplemented by accident without the user of the API noticing it.

By adding a property which allows the generation of default methods which return HTTP 501 developers can opt-in for this behavior.

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.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@andriy-dmytruk