docs(types): Add context to GraphQLRequest type

[Nighthawk14]

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• Adding context to GraphQLRequest type so we can properly use context inside middleware:

const middleware = ({ request, options }, next) => {
  // Here request.context is not available in TS as request is of type GraphQLRequest
  next();
};

[apollo-cla]

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[Nighthawk14]

@evans Bumping up this PR :).