[JAVA] Only add the JsonCreator import when the file contains an enum

[FieteO]

Only add the import for the @JsonCreator annotation when the model actually contains it.
Currently this import is being added to every generated model, even though it is only needed when a model contains an enum.

This is a general annoyance that could be fixed. In my reasonably large backend service there are 7000 problems, with ~ 6500 coming from unused imports in generated code.

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.
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  ./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.
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@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

[FieteO]

The import does not seem to be correctly added for the jersey and feign clients. I still need to investigate, but my first guess is that they are maybe not setting the hasEnums property that is used here as a filter. Edit: That seemed to be a local problem I had with the maven dependencies