Fix `PasskeysControllerConcern` to allow the response to be customized

[tcannonfodder]

Right now the actions are always redirecting to root_path. It should be customizable; maybe with special methods:

• respond_to_after_create
• respond_to_after_destroy

[wilg]

Ran into this today, too. I'm happy to fix it and make a PR, but I'm not sure if contributing to this repo is the active spot for an official Devise integration?

[tcannonfodder]

Hey Wil!
Thanks so much for offering to help out here!! As of right now, this is the place to contribute; devise is not ready to add passkey support to the library itself yet.

Feel free to start a PR for this; and thanks again!!