✨ Remote loading of pages for <amp-next-page> v2

[wassgha]

Changes

• Implements infinite loading of pages from a remote server (specified by the src attribute) and uses a pointer-approach to handle infinite scrolling. The component expects the following format as a result of the server query:

{
   "pages": [...],
   "next": "https://nexturl.com/..."
}

• Introduces an attribute named deep-parsing (off by default when a remote url is present) that controls whether <amp-next-page> should be looking for inline suggestions inside loaded documents (recursively queue pages parsed from <amp-next-page> inline config)
• The current behavior is to use suggestions from both the inline configuration and the remote configuration then, if a remote configuration is present, the server will be responsible for feeding more suggestions, otherwise recommendations will come from parsing the inline configuration of the embedded pages (if present).
• Adds required unit tests for the changes

[caroqliu]

LGTM, Approved once all comments are addressed