Allow auth plugins to configure the required entities' fields

As for now, auth plugins are using hard coded auth entity's fields, like username, and password.
We want to expose these fields (per plugin) to the plugin setting.
E.g.
{
"userFieldName":"username",
"passwordFieldName": "password"
}
By default, these fields will be filled with the relevant fields in our default User entity.

In case the auth entity does not contain the defined fields, the build will be failed with the appropriate error (pointing the user to the settings of the relevant plugin).