Factor out `validates_secure_password`

[jonathanhefner]

Follow-up to #45487, which was reverted by #45753.

This factors validates_secure_password out of has_secure_password, to provide an API for conditionally requiring a password. For example:

class Account
  include ActiveModel::SecurePassword

  attr_accessor :is_guest, :password_digest

  has_secure_password validations: false
  validates_secure_password unless: :is_guest
end

account = Account.new
account.valid? # => false, password required

account.is_guest = true
account.valid? # => true

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/cc @liljack, @dhh