GraphQL: add TagCreate mutation

What does this MR do and why?

Contributes to #509584 (closed)

Problem

The GraphQL mutation to create a tag is missing.

Solution

Add TagCreate mutation to GraphQL to support tag creation.

References

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• GraphQL: add TagCreate mutation (#509584 - closed)

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How to set up and validate locally

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mutation {
  tagCreate(input: {projectPath:"gitlab-org/gitlab-test", name:"new_tag", ref:"master", message:"New tag"}) {
    tag {
      name
      message
      commit {
        id
      }
    }
    errors
  }
}