[cpp-ue4] Series of fixes for cpp-ue4

[Kahncode]

Hello,

After recently updating openapi-generator for our project I noticed some regressions in past pull requests as well as missing features.
Additions are detailed in the commits but:

• Fixed compilation warnings
• Fixed compilation error on APIs using files in http forms, which was a big breaking change and honestly i'm surprised nobody reported it before
• Added support for enums as url parameters
• Cosmetic improvements to the generated code

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.
• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date.sh
  

  Commit all changed files.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.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.

[Kahncode]

@wing328 Please note that I have not updated the samples

[wing328]

samples updated via 3d7c173

[Kahncode]

@leith-bartrich As this was merged, could you please confirm that your use-case is still functional? I want to make sure we didn't break it for your usage. Thanks.