amp-img: Support both native (viewport based) and container-based srcset selection

[aghassemi]

The container-based srcset selection is useful for use-cases such as thumbnail grids and since everything in AMP has a known layout-size and images are lazy loaded, we have a unique opportunity to support this whereas the web platform can not without sacrificing early prefetching of images.

Please note that we may not always honour container-based srcset and may default back to native for certain images above the fold where we want to prefetch event before AMP runtime calculates the layout.

Suggestion is to make this opt-in on amp-img

1- srcset-selection=<native|optimized> (defaults to native)
2- srcset-selection=<native|container> (defaults to native)
3- ???

Related issue: #11575

[aghassemi]

Closing, based on the Design Review linked above, decision was just to polyfil sizes instead which will be done as part of #11575