Create Environment Graphql mutation

What does this MR do and why?

This MR adds Environment Create GraphQL Mutation. It's exactly the same format with RestAPI.

This is later extended for Allow to Select Agent in Environment setting page (#396407 - closed).

Screenshots or screen recordings

GraphQL mutation request:

mutation {
  environmentCreate(input: { projectPath: "root/deployment-approval-demo",
      name: "other"
    }) {
    environment {
      name
    }
    errors
  }
}

Response:

{
  "data": {
    "environmentCreate": {
      "environment": {
        "name": "other"
      },
      "errors": []
    }
  }
}

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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