FIE (a4a) and main document have to allow version isolation. I.e. the main doc and a FIE doc should be able to include different versions of the same extension without conflicting with each other.

• Propagate version info in the binaries
• Enable version argument for all extension APIs
• Modify FIE code to accept versions
• Modify A4A to propagate versions to FIE
• Stretch goal: resolve custom element implementation classes from the ampdoc, not window.