[cpp-restsdk] Enabling forward declarations also for cpp-restsdk generator

[mvistein]

Summary

Enabling forward class declarations also for cpp-restsdk generator as introduced in #6654

Details

This PR modifies only the templates used for cpp-restsdk and is a simple copy&paste from #6654. The petstore sample client does not change since it does not contain circular references.

Technical commitee

@ravinikam @stkrwork @etherealjoy @MartinDelille @muttleyxd

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. 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
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[wing328]

CircleCI failure already fixed in the master.