Allow ability to override userAgent in scenario where using Playwright as engine

It would be helpful to be able to supply a custom User-Agent string to allow for a custom userAgent string to be supplied during the setup of the browserContext (see: https://playwright.dev/docs/emulation#user-agent). Many applications of this (eg checking for behaviour where UA is scanned for old/unexpected UA strings, custom UA strings to avoid adding to Google Analytics/Tag accesses, etc).

It looks like most of the work is done for this change within https://github.com/garris/BackstopJS/pull/1398 (bar resolving merge conflicts) - I had tested a similar solution locally. Can this be reviewed/merged in please?