[Java][Native] Add asyncNative option

[UkonnRa]

Related Issue

#4715

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[UkonnRa]

@lwlee2608 Add the support of async native Java Client

[wing328]

cc @bbdouglas as it's related to Java 11 Native client.

[wing328]

Instead of adding another option. What about adding an "Async" version of the function similar to what we've done in other Java client libraries (e.g. okhttp-gson)? e.g. https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/api.mustache#L283

e.g. addPet => addPetAsync (and addPet simply calls addPetAsync and wait for the completion)

[joerg-wille]

@UkonnRa What is the status of the native async patch? Your changes look good.

[wing328]

UPDATE: I've resolved the merge conflicts, update samples and include the native async petstore client in the drone.io CI tests.

[wing328]

cc @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) @bkabrda (2020/01)

[wing328]

CircleCI failure due to outdated doc/samples. Will fix it in a separate commit after merging into master.

[wing328]

@UkonnRa thanks for the PR, which has been merged into master.

When you're free, I wonder if you can PM me via Slack (https://github.com/OpenAPITools/openapi-generator.git) for a quick chat (IM). Thank you.