Improve templates for C++ Restbed

[LukasWoodtli]

The templates now generate classes which have virtual functions that can
be overridden to implement handlers. This removes the necessity to edit generated files.

See also #273

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.
• ([x]) File the PR against the correct branch: master (5.3.0), 6.0.x

I'm not sure if this is the right version for that change. Please let me know what you think. The generated C++ code is not backwards compatible.

• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

Technical Committee for C++: please have a look at that and provide suggestions
@ravinikam
@stkrwork
@etherealjoy
@MartinDelille
@muttleyxd

[wing328]

Thanks for the PR.

I've changed the PR to target against master, which is the upcoming 5.3.0 release.

Can you please rebase to resolve the merge conflicts?

[wing328]

Thanks for the PR. Let's give it a try by including it in the upcoming v5.3.0 release.