add multipleOf to CodegenProperty (#2192)

[yasammez]

This includes the missing property mapping from #2192. I testet the codegen with a custom template and it worked. But since this is my first PR in this repo, please review carefully if I managed to adhere to all standards and tell me if something is missing.

PR checklist

• Read the contribution guidelines.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[yasammez]

Thank you for noticing. Should be fixed now :-)

[wing328]

The CircleCI failure is not related to this PR.

[wing328]

I've filed #5021 to add multipleOf to various Codegen classes.