[FEAT]: Add github_repository_topics resource

[felixlut]

Describe the need

In my org we currently don't have all repos inside of Terraform, but we want to be able to control the repository topics on them to enforce certain standards. Currently setting the topics via Terraform is only possible when using the github_repository resource, which is too intrusive for my usecase (unless I make some magic with the ignore_changes argument).

Another use-case is also that you can use this resource to set some topics from a platform prospective, while still allowing your users the freedom to add their own topics on top. See example below

data "github_repository" "test" {
  name = "my-cool-repo"
}

resource "github_repository_topics" "test" {
    repository    = data.github_repository.test.name
    # this result in all topics that was manually put on the repo + the standard ones the platform team wants
    topics        = concat(data.github_repository.test.topics, [lower(data.github_repository.test.primary_language), "owner-team", "product-name"])
}

output "name" {
  value = github_repository_topics.test.topics
}

There are API endpoints for doing this: https://docs.github.com/en/enterprise-server@3.10/rest/repos/repos?apiVersion=2022-11-28

I can work on it

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