Optimizer: generate `sizes` attribute as part of image optimization.

Currently, image optimization does not generate a sizes attribute. We should investigate whether there's a good approach to generate a `sizes` attribute as part of image optimization.

//cc @patrickkettner

Start by tracing the image optimization entry point in AMP Toolbox and determine how responsive image attributes are currently handled. Investigate a suitable way to generate a sizes attribute; the work is complete when image optimization reliably emits an appropriate sizes attribute and the behavior is covered by the relevant tests.

Written by the indexing model from the issue text.