chore(bidi): add support for emulation.setUserAgentOverride

[hbenl]

Fixes #37602 and the following tests in Firefox:

• tests/library/browsercontext-device.spec.ts:
  • "should work"
• tests/library/browsercontext-user-agent.spec.ts:
  • "should work"
  • "should work for subframes"
  • "should emulate device user-agent"
  • "should make a copy of default options"
  • "custom user agent for download"
• tests/library/defaultbrowsercontext-1.spec.ts:
  • "should support userAgent option"

[github-actions]

Test results for "tests 1"

3 flaky

⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:1079 › cli codegen › should not throw csp directive violation errors `@firefox-ubuntu-22.04-node18`
⚠️ [firefox-page] › page/page-event-request.spec.ts:182 › should return response body when Cross-Origin-Opener-Policy is set `@firefox-ubuntu-22.04-node18`
⚠️ [chromium] › headerView.spec.tsx:49 › should toggle filters `@web-components-html-reporter`

46932 passed, 821 skipped

---

Merge workflow run.