Allow to not set a totp label upon creation of TOTPInterface

[pkly]

Description

Hello, I've implemented this in our application and it works fine (albeit it is extremely glued to the idea of being a symfony firewall listener, which is not 100% what I was going for), but after creating TOTP provisioning uris and limited testing I noticed some of our employees pointing out the added labels.

It would be nice if we could disable it on the bundle level (just needs a small option in config + TotpFactory change), I'll remove it on our end via a composer patch.

Basically it looks like this when used in Google Authenticator:

App: {identifier, random text}@App

So yeah, not ideal. It wasn't really clear what exactly the TotpAuthenticationUsername is used for anyway. Seems like it's literally just for the label, which is not ideal. Should maybe be renamed to getTotpLabel and allowed to return null to disable, if that's the case?

edit: It appears that without the label TOTP provisioning fails. Still, weird. It seems like it should not be needed if there's an issuer, as that is already a label.