Added HttpClient5 flavour to Java OpenFeign client generator.

[Knuf]

Quick'n'(not-so)dirty (imho) solution to this issue:
#18158

Successfully tested with sample generated from java-feign-hc5.yaml and through Maven with plugin, with a similar configuration:

<plugin>
  <groupId>org.openapitools</groupId>
  <artifactId>openapi-generator-maven-plugin</artifactId>
  <executions>
    <execution>
      <goals>
        <goal>generate</goal>
      </goals>
      <configuration>
        <inputSpec>${project.basedir}/src/main/resources/xxx.yaml</inputSpec>
        <generatorName>java</generatorName>
        <configOptions>
          <useJakartaEe>true</useJakartaEe>
          <dateLibrary>java8</dateLibrary>
          <library>feign-hc5</library>
      </configOptions>
      </configuration>
    </execution>
  </executions>
</plugin>

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

[wing328]

thanks for the PR

please use spaces instead of tabs for indention in the templates.

[Knuf]

@wing328 Yeah, sorry about that, my mistake.

[wing328]

please add the new folder to the github workflow so that the CI will test the change moving forward

.github/workflows/samples-java-client-jdk11.yaml

[wing328]

please also regenerate the samples (step 3, run the update commands twice) and commit the new files

[Knuf]

@wing328 Sorry (again), when I saw that Windows users specific warning I immediately thought all the text below was entirely dedicated to them, so I skipped to next check right away, lol.

[wing328]

please review the CI failure when you've time

[Knuf]

@wing328 Looks like there is a conflict with the concomitant fixes in the crystal sample client from PR #21011:
https://github.com/OpenAPITools/openapi-generator/actions/runs/14219428304/job/39843818657

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/14237406707/job/40119496619?pr=21007

please use spaces instead of tabs in modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java: