Add a new --save-har=my.har option to save requests to HAR file

[asg017]

Playwright has support for HAR files, which saves all network requests during a session to a custom file format.

The API is awkward but we can use route_from_har. We'll need to add update=True to save requests to the file, instead of "serveing" them.

There's a few more options to:

• update_content: "embed"|"attach". Maybe --har-content=embed?
• update_mode: "full"|"minimal". Maybe --har-mode=full?