Fire ValidTwoFactorAuthenticationCodeProvided Event when 2FA session is authenticated

[stefanzweifel]

In an app I'm working on we would like to store a timestamp in the session, when the user last verified their 2FA[^1]. While source-diving I didn't find an event that would be fired, when the 2FA code was verified. (There are only events for when the verification failed.)

This PR adds a new event that is being fired when a new session is being authenticated using 2FA. Adding an event listener for our usecase is now trivial.

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I was contemplating if this event should be fired within the TwoFactorAuthenticationProvider, but as no other events are fired within that implementation I think the controllers is the right place for them.

[^1]: In our app, the admin panel is protected with a RequireTwoFactorAuthentication-middleware that periodically asks users to confirm their 2FA code. (Similar to the \Illuminate\Auth\Middleware\RequirePassword-middleware). We currently run into an UX-issue where folks can't access the admin dashboard immediately after login, as the same 2FA code doesn't seem to be accepted by the TwoFactorAuthenticationProvider within the 30s window after login.

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[taylorotwell]

I would call the event something like ValidTwoFactorAuthenticationCodeProvided

[taylorotwell]

Please mark as ready for review when the requested changes have been made.

[stefanzweifel]

@taylorotwell Definitely sounds and feels better than my original name. Thanks!