[keycloak-dev] Introduce option to select username mode for a realm

We've have someone from the community that wants to use mobile number as the username, as well as verify mobile number by sending a code via SMS. See "Login by mobile number" thread in user mailing list for more details. They are also willing to contribute this back to the community. That made me think it may be nice to be able to configure the behavior of the username "field" for a realm. We could have a simple drop-down in the admin console to configure username mode, with the following options: * Username/email - default behavior where a user provides both a username and email, and the user can login with either. In this mode email has to be unique. * Username - a user can only login with a username. In this mode we could relax the requirement that email has to be unique (that may be difficult though as it would require not using a database constraint, which may make it rather difficult to guarantee uniqueness in other modes) * Email - in this mode only email can be used to login. In this mode username field would not be displayed on the registration form or account management console. In the token the username would be set to email. In this mode verify email address should be enabled by default. * Mobile - user logs in with a mobile number. We can either just add mobile number to the username field or add a new mobile field and require uniqueness on that field. In this mode verify mobile number should be enabled by default. With regards to implementation I think it would be easier to make the existing username/password authenticator, registration form and account management adopt to the mode rather than have separate authenticators, etc.. for each mode.