Add `why` field to GraphQL Errors

# TL;DR:
This RFC proposes adding the `why` field to GraphQL errors that contain a list of reasons why this error happened.

## Problem Statement

Sometimes, during execution multiple errors happening at the same time (e.g. parallel execution) or to provide full context on what happens you need to chain errors.

Recent changes in JS, added two features `AggregatedError` and `Error.cause` to solve both problems but we don't have any mechanism to represent this additional error context in graphql.

But I don't think this proposal is JS specific and other languages have also mechanisms for aggregated errors and chains of errors.

** TBD: Examples in other languages **

## 🧑‍💻 Proposed Solution

Add ability to attach an array of sub-errors to graphql error under a property named `why` (the name is up to discussion).
With following limitations:
1. `why` is an optional non-empty array of objects
1. Each individual object should contain only `message` property (required) and `extensions`(optional)

## 🎬 Behavior

Error chaining:
```
{
"errors": [
{
"message": "Expected value of type "DateTime", found "07/11/2021"",
"locations": [ { "line": 6, "column": 7 } ],
"why": [{
message: "Invalid date-time please specify in 'yyyy-MM-dd'T'HH:mm:ss.SSSZ' format"
}],
}
]
}
```

Aggregated errors:
```
{
"errors": [
{
"message": "Failed to fetch all necessary data",
"why": [
{
message: "Failed to fetch from https://example.com/api/resource/1"
},
{
message: "Failed to fetch from https://example.com/api/resource/2"
},
],
}
]
}