Add topic resource support, third trial

[eliobischof]

Description

Cherry picked the commits from #832 that has a messed up commit history.

PR Checklist

• Resource attributes match 1:1 the names and structure of the API resource in the GitLab API documentation.
• Examples are updated with:
  • A *.tf file for the resource/s with at least one usage example
  • A *.sh file for the resource/s with an import example (if applicable)
• New resources have at minimum a basic test with three steps:
  • Create the resource
  • Update the attributes
  • Import the resource
• No new //lintignore comments that came from copied code. Linter rules are meant to be enforced on new code.

[timofurrer]

Thanks @eliobischof for creating yet another PR for this! 🎉

I'm still super unhappy about the tradeoffs we had to make here (absolutely not your fault @eliobischof :D )- but for the lack of a better solution I think we can merge this.

I think I'll follow up with some changes in go-gitlab (xanzy/go-gitlab#1381) to support removing the avatar (since https://docs.gitlab.com/ee/api/topics.html#remove-a-topic-avatar) is available and also include the total_projects_count and avatar_url as Computed attributes. But it shouldn't block that PR from finally going through :)

[armsnyder]

Looks good! Just some typos and style things.

[timofurrer]

I took the liberty and applied @armsnyder suggestions. Once the checks pass I'll merge!

Thanks @eliobischof for your effort ❤️ much appreciated 🎉

I've applied your suggestions 🎉

[github-actions]

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