Add separate render function to CheckoutProgressBlock

[MatthieuScarset]

Problem/Motivation

As a site builder, I would like to display the CheckoutProgressBlock on the Cart page (e.g /cart).

As of today, I can not do so. This block is restricted to checkout pages, where the commerce_product parameter is available.

Proposed solution

To separate the rendering of the CheckoutProgressBlock in another function. Currenlty, everything is inside the build() method.

This way, as a developer, I can create a child block and render the CheckoutProgressBlock with my own logic.

Example

Here is an example of a child block which load the current Cart if the default CheckoutProgressBlock rendered nothing.

https://gist.github.com/MatthieuScarset/c29672ee99805d955988bc150bb642ab