Add optional function to modify headers

[chdanielmueller]

Adding an optional function which can modify the headers before they are being sent.
It allows to add additional headers or modify existing headers like Authorization.

It can be used like this:

new window.EmbeddedSandbox({
  target: '#sandbox',
  initialEndpoint: window.location.href,
  initialState: {
    sharedHeaders: {
      'X-SpecialHeader': 'WILL_BE_REPLACED_AUTOMATICALLY'
    }
  },
  modifyHeaders: async (url, headers) => {
    // Add Authorization header if not provided
    if (!headers.Authorization) {
      const accessToken = await window.getAccessToken();
      headers.Authorization = 'Bearer ' + accessToken;
    }
    // Modify special header if shared header is not unchecked
    if (headers['X-SpecialHeader'] === 'WILL_BE_REPLACED_AUTOMATICALLY') {
      const special = await window.getTheValue();
      headers['X-SpecialHeader'] = special;
    }
    return headers;
  }
});

closes #308
replaces #307

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🦋 Changeset detected

Latest commit: 4e113e7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@apollo/sandbox	Minor

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