Getter and Setter for hideGenerationTimestamp

[jmini]

Work in progress for #7997.

[jmini]

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\. => this is not necessary because no generated code is affected. Unit tests were created to ensure no regression.
• 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. => I am not sure who is concerned by this technical change.

Description of the PR

See: #7997

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I do not think that I am responsible for Build FAILED in CircleCI. The main commit eb3abda1242306ebf3f6226fe9963f65ef5c5785 has 4 successful checks.

[wing328]

@jmini thanks for the PR, which looks good to me.

[jmini]

Thank you for the merge of this PR. When I wanted to use it in my project, I have noticed, that the setter und getter should also be available on the Interface: I have opened #8009 for that.