fix(toolbar): Include credentials with fetch requests

[ryan953]

Our fetch requests are cross-domain by design, we're making requests from a customer domain like acme.sentry.io into a an region like us.sentry.io. Therefore we need to include credentials. This is also how the website is configured:

sentry/static/app/api.tsx

Line 338 in aa22f5d

this.credentials = options.credentials ?? 'include';

More docs: https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch#including_credentials

Previously we had same-origin which worked because we weren't using the regionUrl. We were instead sending api requests to the customer domain at acme.sentry.io. This meant that api requests were slower than using the regionUrl, but and also we were unable to make requests for api endpoints that don't include an /:organization/ segment.

So using the regionUrl is an improvement, and this PR updates the credentials field to match.

Another thing to consider is that before we were using window.location.origin to make requests. That has since been replaced by

sentry/src/sentry/templates/sentry/toolbar/iframe.html

Line 31 in aa22f5d

const regionUrl = '{{ region_url|escapejs }}';

so we can trust that we're always sending these credentials off to a domain that the server trusts and told us about.

Related to #81942

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

Additional details and impacted files

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