#7185299 Add Cart Variant Dropdown Selector for A/B Test

Changes proposed in this Pull Request

• Restructure <ItemVariantPicker /> component to render either radio buttons or dropdown.
• Add A/B Test calypso_jetpack_cart_with_dropdown_variant_selector_2 to Calypso to control which variant picker type is used.
• Miscellaneous supporting changes.

Testing instructions

1. Boot branch as Calypso Blue
2. Add a product for a Jetpack Site to the cart
3. Verify the cart matches production in design
4. Use the Abacus link from pbxNRc-1rJ-p2 to add yourself to the treatment of the test
5. Refresh the cart and verify it now matches the design ( iIYWCXVJSm06ZfzbeEWUT0-fi-1834%3A23596 ) for the test:
6. Test the functionality of the dropdown selector
   • Clicking and keyboard controls should work
   • Space should open the sector, enter should select an option