FlareSolverr: Provide a way to select captcha solver

[keldian]

FlareSolverr now has two captcha solvers, hcaptcha-solver and harvester. Right now the community role has the former in the CAPTCHA_SOLVER env. Perhaps make a config file available for the user to select which solver to use (or none), as well as a HARVESTER_ENDPOINT uri.

https://github.com/FlareSolverr/FlareSolverr#captcha-solvers