[cpp-ue4] Improved retry system to use Unreal's FHttpRetrySystem

[Kahncode]

Hello, I have improved the retry system of the generator as per Epic's suggestion. The new Http retry system allows timeouts and exponential backoff (technically unreal's implementation is arithmetic not exponential).

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.
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  ./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.
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[Kahncode]

Hello, I have not managed to produce updated cpp-ue4 samples using the script. I'd be happy to fix what is needed if someone can guide me.

[wing328]

Updated samples via 47693b8.

[Kahncode]

@wing328 it seems like the failed check is about the contributing.md change? anything I should do?

[wing328]

Don't worry. I'll fix it after merging this PR to the master.

Thanks for the PR as always.

[wing328]

Pushed 7703c92 with updated doc to the master.