feat(set-context): create set context exchange

[JoviDeCroock]

Resolves #2406

Summary

Creates the context exchange used to automatically resolve async/sync context setters.

import { createClient, dedupExchange, cacheExchange, fetchExchange } from 'urql';
import { contextExchange } from '@urql/exchange-context';
const client = createClient({
  url: 'http://localhost:1234/graphql',
  exchanges: [
    dedupExchange,
    cacheExchange,
    contextExchange ({
      getContext: async () => {
        return await getToken();
      },
    }),
    fetchExchange,
  ],
});

purposefully added no changeset as it will be picked up by the release action as initial release

[changeset-bot]

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Latest commit: f91d9a8

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