browser: add chromium.connectOverCDP for external browsers

[pkiv]

What?

Scripts can connect to external CDP-compatible browsers at runtime without declaring options.browser.

import { chromium } from 'k6/browser';

export default async function () {
  const browser = await chromium.connectOverCDP('ws://...');
  const page = await browser.newPage();
  // ...
  await page.close();
  await browser.close();
}

The naming follows Playwright's chromium.connectOverCDP().

Why?

Users with their own browser infrastructure want to run k6 browser tests against their own browsers. Their flow requires a fresh WebSocket URL per session, obtained at runtime via an API call.

Note

CI failures are not related to this PR.

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Re-opened from #6005 (original work by @inancgumus); commit authorship preserved.

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❌ pkiv
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