Add screencast functionality

[bradleyscollins]

This pull request adds an API that exposes CDP's screencast capability.

See the following:

• Page.startScreencast
• Page.stopScreencast

Note: At this time, the Chrome DevTools Protocol documentation still has screencast methods/types marked as "experimental."

[bradleyscollins]

A number of the test failures are on account of a difference between expected vs. actual screenshot height.

We noticed that as of Chrome 128, the heights of our screenshot/screencast images were different from (less than) the height specified in the window_size option of the Browser constructor. We had to set the viewport size explicitly to get the expected screenshot height.

[route]

@bradleyscollins could you please rebase on master? after that we can merge, should be fixed in there

[bradleyscollins]

@route, I think I have rebased correctly (and fixed a brittle test). Just let me know if there's anything else you need me to do.

And thank you for your kind consideration of my pull request.

[vojtad]

Hello, is there anything I can do to help to get this merged? We would like to use the Screencast API for debugging flaky tests. Thanks!

[route]

Thank you!

[bradleyscollins]

Yes, indeed, my thanks to you as well!