bext: add firstSourcegraphUrl to logs

Created by: taras-yemets

Closes #32249 (closed)

Adds firstSourcegraphUrl to logs when default Sourcegraph URL is used.

Notes

• tested in Chrome
• telemetry is disabled for Firefox, so logs are not sent at all, see:
  • client/browser/src/shared/code-hosts/shared/codeHost.tsx
  • client/browser/src/shared/util/optionFlags.ts

Test plan

1. Run sg run bext for dev version or yarn --cwd client/browser build for prod one.
2. Set Sourcegrapg URL value to sourcegraph.com.
3. Open a file on a code host (for example, this one).
4. Open browser extension background page.
5. Perform an action being logged (for example, hover on a token to see a code intel popup).
6. Inspect logEvent requests on the network tab of browser extension background page.
7. Ensure that firstSourceUrl value is present in the request payload.
8. Change Sourcegrapg URL value (for example, to k8s.sgdev.org).
9. Repeat steps 5 and 6.
10. Ensure that firstSourceUrl value is not present in the request payload.