[WIP] Optimize images as part of production builds

[parlough]

trafficstars

Closes https://github.com/flutter/website/issues/6487 Closes https://github.com/flutter/website/issues/9527 as this plugin also validates alt is always specified

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 77bd8a6da1a08366ffe4011c6284778ef2c48466):

https://flutter-docs-prod--pr10391-feat-optimize-images-9kqzqpc9.web.app

[sfshaza2]

Parker, can we either land this or close? I really hate stinky PRs that hang around for weeks and weeks.

[parlough]

@sfshaza2 Yep, sorry! I was waiting for the dart.dev equivalent to land to work out the kinks there first. It landed today with https://github.com/dart-lang/site-www/commit/bcbff3ea28c25a6d2fb44b2e5e3b761c704a4a7f.

I'll work on getting this updated and ready for review sometime this week.