Added a property to the Spring generator to avoid the use of the `ResponseEntity` type

Created by: rcardin

The PR adds the boolean property useResponseEntity to the Spring generator. Through the property, it's to generate controllers and APIs that return directly a value, avoiding its wrapping inside a ResponseEntity.

The PR fixed issue #6135.

I cannot understand how to add automatic tests verifying the generated files. Please, help me with some hints!

As requested, I tag the Java language technical committee: @bbdouglas, @sreeshas, @jfiala, @lukoyanov, @cbornet , @jeff9finger, @karismann, @Zomzog, @lwlee2608

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.
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