ISSUE: #20306: Add support for the async implemenation of the Java apache httpclient5 libary (apache-asynchttpclient)

[jbarth]

This change adds template files for the apache http client 5 with async support. Unit test to verify generator has been added, and these templates have been used in private project to prove the changes. These are minor changes to the template files to return CompletableFutures in place of the blocking responses.

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 || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (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.x.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.

Tagging:
@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @martin-mfg

[wing328]

@jbarth thanks for the PR

when you've time next week, can you please PM me via https://join.slack.com/t/openapi-generator/shared_invite/zt-2uoef5v0g-XGwo8~2oJ3EoziDSO1CmdQ to further discuss this PR?

[jbarth]

@jbarth thanks for the PR

when you've time next week, can you please PM me via https://join.slack.com/t/openapi-generator/shared_invite/zt-2uoef5v0g-XGwo8~2oJ3EoziDSO1CmdQ to further discuss this PR?

Hi @wing328 this slack link doesnt work - please can you provide a new one. Thx

[wing328]

can you please try https://join.slack.com/t/openapi-generator/shared_invite/zt-2wmkn4s8g-n19PJ99Y6Vei74WMUIehQA ?