[firebase_auth] Support disabling reCaptcha on Web

[itnks]

The previous ticket [1942] that asked for this feature was closed. The last release [0.18.1+1] only Implemented this feature on iOS and not the web platform. It would be very nice if we could skip the reCaptcha step to improve the development experience.

[brampersandon]

++, for various reasons, the ReCaptcha implementation here is really onerous. Even as a dev, I can't pick all the fire hydrants, bicycles, or buses in frame! There has to be a way to tune this to reduce friction.