A single function that takes GraphQL parameters and does parsing, validation, context assembly and execution/subscription

Envelop is currently split into parts and requires the user to integrate them in order every time they want to use envelop and have a valid GraphQL execution chain.

In essence, the redundancy is:
```js
const { parse, validate, contextFactory, execute, schema } = getEnveloped({ req })

const { query, variables } = JSON.parse(payload)

try {
const document = parse(query)
} catch (err) {
return { errors: [err] }
}

const validationErrors = validate(schema, document)
if (validationErrors.length > 0) {
return { errors: validationErrors }
}

const context = await contextFactory(req)
const result = await execute({
document,
schema,
variableValues: variables,
contextValue: context
})

return result
```

In order to properly process a GraphQL request, you'll always have to parse, validate and execute/subscribe; with this in mind, I suggest a single, unified, `perform` function that does all of the above:

```js
const { perform } = getEnveloped({ req });

const { operationName, query, variables, extensions } = JSON.parse(payload);

const result = await perform({ operationName, query, variables, extensions });

return JSON.stringify(result);
```

This approach, not only simplifies the usage of envelop, but additionally allows manipulation of the final result through an accompanying `onPerform` hook (including parsing and validation errors) to inject implementor specific requirements - like the `extensions` field for Apollo's FTV1 tracing.

Supersedes #1491
Necessary for #1490