Delete environment mutation

Why

See GraphQL Mutation: Stop and Delete Environment (#388206 - closed) for more context

What

• add a new graphQL mutation to destroy an environment
• a new delete service

Local test

You'll need an Environment (I just used Environment.last)

• start gdk
• bundle exec rails c
• e = Environment.last
• e.state
• Make sure environment state is already stopped (otherwise policy prevents deleting)
• copy e.id
• go to https://gdk.test:3443/-/graphql-explorer or wherever your graphiql is
• run something like

mutation {
  environmentDelete(input: { id: "gid://gitlab/Environment/19"}) {
    errors
  }
}

you should see an output like

{
  "data": {
    "environmentDelete": {
      "errors": []
    }
  }
}

verify environment is destroyed