Need to be able to set client certificates in browsers.

[teodesian]

https://github.com/microsoft/playwright/issues/1799#issuecomment-731734494

suggests there is indeed a way. The core problem I see is that you’d want the file read to get done in JS rather than passing bytes on from perl, as those tend to choke JSON encoders/decoders.

I’ll need a mechanism to hint playwright_server that it ought to read a particular file parameter.