Add authentication for screenshots

[tacman]

Description

If I want a screenshot of /my-profile, the user needs to be logged in.

Example

bin/console pwa:create:screenshot https://127.0.0.1:8003/my-profile

I think panther has some support for this, but I don't really know. Maybe

bin/console pwa:create:screenshot https://127.0.0.1:8003/my-profile --username=tac --password=my.password

[tacman]

As an alternative, how about the option to pass a header when taking the screenshot, which could include some sort of cookie or authentication. It's messier, of course, but adding login is probably more complicated than it's worth.

If #138 worked, I'd probably take the screenshots locally and disable security, though that's not very feasible either. Passing a header, though clunky, would work.