issue-19600: Add nullability annotations to Java generated clients

[nvervelle]

Motivations:
Have generated clients properly annotated for nullability to be able to check code using them with tools like NullAway (#19600)

Modifications:

• First commit :
  • Add nullable_var_annotations template to handle nullability annotation on vars
  • Adapt pojo templates to use the nullability template (except microprofile)
  • Adapt tests
• Second commit :
  • Modifications from export_docs_generators.sh

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/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  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*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.6.0 minor release - breaking changes with fallbacks), 8.0.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 (Java: @bbdouglas, @sreeshas, @jfiala, @lukoyanov, @cbornet, @jeff9finger, @karismann, @Zomzog, @lwlee2608, @martin-mfg)

[martin-mfg]

Hi @nvervelle, thanks for the PR!

For feign, the annotations on the field are currently only generated if the field is not a container. Moving the {{>nullable_var_annotations}} to line 75 should fix that. Other than that the PR looks good to me.

[nvervelle]

Hi @martin-mfg !

Thanks for the comment. I modified the template for feign and updated the PR accordingly (I edited each commit so that I still have one commit for the manual changes, and one commit for the changes made by the script)