Add Rails 8 authentication generator

[jeromedalbert]

Follow-up to rspec/rspec-rails#2811.

Problem

When running bin/rails generate authentication for a Rails 8 app that has the rspec-rails gem, generation ends with the following error: error factory_bot [not found].

This is because rspec-rails' AuthenticationGenerator contains hook_for :fixture_replacement, which calls factory_bot_rails, but it errors out since factory_bot_rails does not have an authentication generator.

Solution

Add authentication generator that creates a user factory with the email_address and password_digest fields. These fields come from the Rails authentication generator.