Payment method component for paid signup flow

This provides the payment method component for the paid signup flow

This MR is not functional on it's own, but instead is created in an effort to split up gitlab-org/growth/product#87 (closed) into smaller, more digestable MR's.

Design: https://gitlab-org.gitlab.io/gitlab-design/hosted/kcomoli/growth%2387-gitlab-com-purchase-experience--specs-preview/#artboard13

There is no changelog, since the feature is behind a feature flag.

To see the previous steps in development setup:

• Enable the feature flag in the console: Feature.enable(:paid_signup_flow)
• Add return [{id: 'x', code: 'bronze', price_per_year: 48}] on ee/app/helpers/subscriptions_helper.rb#16 (to avoid having to configure and run the customers app)
• Sign in and visit http://localhost:3000/-/subscriptions/new

The current step is not viewable without configuring and running the customers app, unfortunately.

Closes gitlab-org/growth/engineering#5051 (closed)