Convert Shopper Multi Currency Widget spec to Playwright

[ismaeldcom]

Fixes #10067

Changes proposed in this Pull Request

• Fix addMulticurrencyWidget, the isWidgetAdded check is looking for a heading that is inside an iframe, we need to first wait until every sidebar loads and then look for the widget iframe.
• Implement REST API createOrder method, to get a pay order link without using the UI, which is faster and less dependent on WC changes. I think there is room for more API utils, and a refactor to use a singleton, but let's leave that for future optimizations.
• Implement Playwright spec
• Remove Puppeteer spec

I've explored the option of using the REST API for adding the widget, to avoid "testing" something outside of WooPayments, but it requires three different calls, making it even slower. Anyway, we could consider doing this differently later, like adding the widget using the CLI in the setup script.

Testing instructions

• Checkout dev/10067-convert-shopper-multi-currency-widget-spec
• Run npm run test:e2e-pw shopper-multi-currency-widget
• Test should pass locally.
• Test should pass in GH actions.

---

• Run npm run changelog to add a changelog file, choose patch to leave it empty if the change is not significant. You can add multiple changelog files in one PR by running this command a few times.
• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

Post merge

• Link to testing instructions from release testing doc following these instructions : QA Testing Not Applicable
• Add or update critical flows and testing instructions for critical flows, if applicable.
• Add what's changed (description, screenshot, demo videos etc.) to the release announcement post, if applicable.