[amp-story-shopping] Allow for a custom cta

[cpauwels]

Description

Similarly to the amp-story-attachment and amp-story-outlink, allow for passing an optional cta-text attribute so that it is possible to override the "Shop Now" text.

Please also update the following:

• Documentation
• amp validation so that cta-text is a valid attribute

[processprocess]

Good idea :) I updated the description to include updating the documentation and validation.

[processprocess]

This needs to follow the API for amp-story-page-attachment
Reference the docs here.

Steps to do this:

1. Allow a new attribute on amp-story-shopping-attachment called cta-text.
2. In buildcallback, if cta-text is defined, put it into the jsx template instead of the translated string.