Improve Apache Httpclient support

[maciex]

What was changed

Apache Httpclient template files moved to proper directory, after this they should work when used with --additional-properties library=apache-httpclient CLI option.
Sample files were regenerated and they really use the apache-httpclient and compile properly (using both Gradle and MVN).

I think this could be merged to all branches where apache-httpclient is present (and probably should be merged, as without this it's not possible to easily use this library).

Before

Before the fix when --additional-properties library=apache-httpclient CLI option was used the default Java library was used.
Also there was no specific build.gradle created to properly compile the code using Apache Httpclient.

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/utils/export_docs_generators.sh
  

  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*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• 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 Technical Commitee, please review:
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) @nmuesch (2021/01)

FYI @harrywhite4

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[maciex]

Commits authors fixed and force pushed.

The circleci PR check failed, but the failure doesn't seem to be caused by the code changes.

[wing328]

Tested locally and the result is good.

Thanks for the PR.