feat: JWTExtendContextFields should receive generics

[jjangga0214]

Hi!

I have a yoga subgraph.
I decode JWT payload from the gateway, and the subgraph consumes it as context.

import { YogaInitialContext } from 'graphql-yoga'
import { JWTExtendContextFields } from '@graphql-hive/gateway'

export interface GraphQLContext extends YogaInitialContext {
  jwt: JWTExtendContextFields
}

And JWTExtendContextFields is declared like this.

export type JWTExtendContextFields = {
  payload: JwtPayload;
  token: {
      value: string;
      prefix?: string;
  };
};

The problem is that JwtPayload is fixed.
So I cannot extend it to contain my own payload properties.
It should be conceptually like this.

export type JWTExtendContextFields<T = {}> = {
  payload: JwtPayload<T>;
  token: {
      value: string;
      prefix?: string;
  };
};