[PP-1] JS minification cache warming

Problem/Motivation

Follow-up from #3302755: On-the-fly JavaScript minification to implement cache warming for minification of js files, this is an intermediate step between the previous issue and #3308122: Pre-minify core JavaScript.

so it really just needs to be a pre-warm step rather than blocking - we'll be minifying admin assets that might not be requested, while the front end should already be trying to serve requests to anonymous users that might only need one js file from a theme or similar. It'll be quicker to do that one file than wait for all minification to finish. I think implementing the caching in JsOptimizer and pre-warming in hook_rebuild() or similar will do exactly what we want from that point of view though.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet