feat(checkout): CHECKOUT-8281 Add 3 New Checkout Extension Regions

[bc-peng]

What?

Add 3 new checkout extension regions.

Depends on

https://github.com/bigcommerce/checkout-sdk-js/pull/2567

Why?

New business requirement.

Testing / Proof

Manual testing

The extension region after the summary section

It will appear at the bottom of the page on the mobile view.

@bigcommerce/team-checkout

The selected shipping method region

It will only show up in the single shipping mode

https://github.com/bigcommerce/checkout-js/assets/88361607/01bc83cd-f6de-4d58-baa9-89edeb704c6e