add header to apollo client code example

Example: apollo client add custom header

import { ApolloClient, HttpLink, ApolloLink, InMemoryCache, concat } from '@apollo/client';

const httpLink = new HttpLink({ uri: '/graphql' });

const authMiddleware = new ApolloLink((operation, forward) => {
  // add the authorization to the headers
  operation.setContext({
    headers: {
      authorization: localStorage.getItem('token') || null,
    }
  });

  return forward(operation);
})

const client = new ApolloClient({
  cache: new InMemoryCache(),
  link: concat(authMiddleware, httpLink),
});