Note: this PR is not complete as there are issues I cannot do on my own (see below), hence draft PR.

It partly fixes the related issue #16974.
This PR simply adds a line to the top of *_api.rs importing the models used as arguments of the endpoint functions.

The issue is that:

1. If a model is used multiple times, it's being imported multiple times, which causes an error,
2. There are warnings that some imports are unused (maybe goes away after fixing 1. above)

As Mustache is a logic-less template engine, to filter out a unique list of models requires delving into the enormous Java codebase of this project, which I can't do.

Mentioning technical committee per PR checklist: @jacob-pro

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 7.1.0 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.