Feature Request: add an option to block query execution

Coming from https://github.com/graphql/graphiql/issues/2721

I am running some validations and would like to block requests on a client side.

Atm, I replace fetcher with custom error message, which does not seem ideal.

```js
const query = queryEditor?.getValue()
useEffect(() => {
const isValid = validateQuery(query)
setExecutable(isValid)
}, [query, setExecutable])

Promise.reject('Query is not executable')
}
/>
```

Maybe extend `useExecutionContext` hook?

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GraphiQL 2.0 is amazing! Thank you for the great work! 🙌