browser: add `page.on('requestfinished')` event handler

[pkalsi97]

What?

Implements the page.on('requestfinished') event handler that fires when a network request successfully completes (receives a response).

• closely followed the implementation of page.on('request') and page.on('response') events.
• Added integration test
• Added example script

Why?

Reasons mentioned in the issue #4300

Checklist

• [x] I have performed a self-review of my code.
• [x] I have commented on my code, particularly in hard-to-understand areas.
• [x] I have added tests for my changes.
• [x] I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

Please do not merge this PR until the following items are filled out.

• [ ] I have added the correct milestone and labels to the PR.
• [ ] I have updated the release notes: link
• [ ] I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• [ ] I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

Closes: https://github.com/grafana/k6/issues/4300 Parent: https://github.com/grafana/k6/issues/4232