data-credentials support for amp-story bookend

[Pateriack]

• Add support for the 'data-credentials' attribute for amp-story bookend.

According to https://www.ampproject.org/docs/fundamentals/amp-cors-requests#utilizing-cookies-for-cors-requests, most components that allow CORS requests allow the developer to optionally set the credentials mode through a 'credentials' or 'data-credentials' attribute. This does not work for amp-story bookend.

Fixes #21598

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CLAs look good, thanks!

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[Pateriack]

/cc @calebcordry @newmuis

[calebcordry]

Thanks for contributing. Just a small nit.

[calebcordry]

I think this could be shortened to const credentials = bookendEl.getAttribute(BOOKEND_CREDENTIALS_ATTRIBUTE_NAME); as it will be null if nonexistent. You may have to change the type below.

[calebcordry]

LGTM! Thanks again!

[Pateriack]

@calebcordry thanks for your input! It appears that one test on saucelabs has failed, and I believe it is unrelated. Could it be flakey tests? Is there any way for me to re-run those tests to see if that is the case?

[calebcordry]

Sauce labs is non blocking, and probably was a flake. I restarted it just in case. I will merged when it finishes.