Adds content to createCommit mutation

What does this MR do?

GraphQL: Adds content to the CreateCommit mutation

mutation{
  commitCreate(input: { projectPath: "root/rocky-horror", branch:"master", message:"science fiction", 
    actions: [
      { filePath: "README.md", action: UPDATE, content: "double feature" },
    {
    content
  }
}

{
  "data": {
    "commitCreate": {
      "content": [
        "double feature"
      ]
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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#324885 (closed)