Adding context length configuration for 2FA to ensure better security standards

[MattLoyeD]

Follow up of #567

Description

We need to ensure a minimum length for 2FA secret, current secret length by default is 80-bit (16 characters), but 128-bit (26 characters) is becoming minimum in some cases and the best default is 160-bit.

It's recommended to use 128-bit or 160-bit because some Authenticator apps may have problems with non-RFC-recommended lengths (Namely https://play.google.com/store/apps/details?id=org.fedorahosted.freeotp).

Implementation

Added some contextual config via :
config('fortify-options.two-factor-authentication.secret-length', 16);

Retro compatible and define a security standard as expected by new RFC.

[taylorotwell]

We don't use snake case variables. We also can't change the contract on a patch release since it would break existing applications. Is there a way to just make this opinionated without breaking anything? Please mark as ready for review when the requested changes have been made.

[MattLoyeD]

Thank you @taylorotwell for your precise review, I made the necessary change. We can probably even make less breaking change by just adding the config(...) in src/TwoFactorAuthenticationProvider.php directly.