Improve GraphQL Variable Validation

[ThisaruGuruge]

Description:
When a document contains more than one operation with variables, variable validation throws an error for not providing variable values for unused variables.

Consider the following schema:

type Query {
  messages: [Message!]
  message(id: Int!): Message
}

type Mutation {
  addMessage(input: MessageInput!): Message
}

type Message {
  id: Int!
  from: String!
  text: String!
}

input MessageInput {
  from: Int!
  text: String!
}

Send the following document to the above service with operationName: "SendMessage" :

query GetMessage($id: Int!) {
    message(id: $id) {
        from
        text
    }
}

mutation SendMessage($input: MessageInput!) {
    message: addMessage(input: $input) {
        from
    }
}

With the following variables map:

{
    "input": {
        "from": 1,
        "text": "Hi"
    }
}

This throws an error:

{
    "errors": [
        {
            "message": "Variable "$id" of required type Int! was not provided.",
            "locations": //...
         }
    ]
}

But this operation can be executed without the $id variable since it is not required for the operation.

IMO, after parsing, we should only validate the given operation from the document, and we can omit the validation of other opertations.