Add feature flag and interface for MFA prototype

[jmdembe]

This is the first step in creating a prototype for multi-factor authentication. The user test will ask users to select and provide multiple authentication methods. After testing is completed, work can/will be done to implement in production.

This PR creates the feature flag and logic on whether or not users will be shown the checkbox selection vs. radio buttons. On testing on this current PR, you should only see the radio buttons as the feature flag is currently set to false.

Future considerations for creating the feature/prototype:

• Would it be worth creating a feature branch to further develop and write tests against?

[aduth]

I expect there might need to be some server-side changes to handle selection as an array, not a single option?

identity-idp/app/forms/two_factor_options_form.rb

Line 16 in 0ea9062

self.selection = params[:selection]

[zachmargolis]

Should we bring back self.selection = Array(params[:selection]).first?

[jmdembe]

@aduth that makes sense to change on the server side
@zachmargolis my thinking was to handle that first option in mfa_redirect_helper.rb. After seeing what helpers do, what I did may not be the best option and as it stands now. I can make your change and use it in a new controller?

[aduth]

We can use AlertComponent to simplify this and ensure consistency:

Suggested change

	<div class="usa-alert usa-alert--info margin-bottom-4" role="status">
	<div class="usa-alert__body">
	<p class="usa-alert__text">
	<%= t('two_factor_authentication.mfa_instructions') %>
	</p>
	</div>
	</div>
	<%= render AlertComponent.new(type: :info, class: 'margin-bottom-4') do %>
	<%= t('two_factor_authentication.mfa_instructions') %>
	<% end %>

[jmdembe]

Breaking this PR out to solely support the feature flag and front-end ticket. LG-5986 details the work of this PR. Data storage and routing will be addressed in LG-5988

[zachmargolis]

LGTM