Added new language: UE4 C++ client

[Kahncode]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Added a client generator for Unreal Engine 4 C++ framework.
During that process I noticed a lot of issues with the abstract cpp generator as well as the existing c++ generators (qt5 and cpprest). Some of these issues are currently fixed within the UE4 generator, but could be easily moved to the base classes. In this PR i focused on making the UE4 generator work with the least possible changes to the codebase. If required, I could move some of my improvements to affect other CPP generators.
Notes:

• I have not been able to run tests locally, I'm very unfamiliar with maven and could not manage to make it work fully.
• I may have also let slip some Java no-nos, again this is a language I do not usually program in, and I welcome every bit of feedback for this.
• I tested the UE4 generator with Petstore and some real-life complex use-cases for my professional project.
• This generator should support more things than the existing C++ generators as far as I can see, most notably it supports enumerations properly.

[HugoMario]

hey @Kahncode let me know once it's ready for merging

[HugoMario]

thanks a lot!