Add SRI support

[botandrose]

Summary

This is a proof-of-concept for adding optional Sub-Resource Integrity support via a new --integrity command line option, and a new :integrity key in config/importmap.rb:

bin/importmap pin md5 --integrity

When this option is enabled, the SRI checksum is calculated and added to config/importmap.rb:

pin "md5", to: "https://cdn.jsdelivr.net/npm/md5@2.3.0/md5.js", integrity: "sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC"

Which ultimately adds it to the HTML importmap, with two ESMS options enabled:

<script type="esms-options">{ "polyfillEnable": true, "enforceIntegrity": true }</script>
<script type="importmap-shim">...</script>
<link rel="modulepreload-shim" href="https://cdn.jsdelivr.net/npm/md5@2.3.0/md5.js" integrity="sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC" />

More discussion here: #122

Drawbacks

• Note that this requires the polyfillEnable: true option to be turned on so that the shim is used for ALL clients. Otherwise, this integrity checking is bypassed when using native implementations, e.g. Chrome. So there is a performance/security trade-off here.
• The previous also requires that we append "-shim" to the relevant script[type] and link[rel] attributes. This breaks some other tooling that is looking for script[type='importmap'] exactly, e.g. { eagerLoadControllersFrom } from "@hotwired/stimulus-loading".

TODO:

• More tests. This is mostly a proof-of-concept spike with only a few narrow unit tests, so I want to make sure that this plays well in all contexts.
• Propshaft integration. Currently failing in CI.
• Investigate whether or not its possible to work with ESMS to avoid appending "-shim" to script[type] and link[rel] attributes? ESMS has progressed since I dove deep last year, maybe this has changed.
• Add documentation

Thoughts?

[dhh]

Version 2.0 is dropping the shim and also vendoring all pins. So I don't think this is relevant any longer. Appreciate you exploring it, though!

[botandrose]

@dhh Ah, yeah, without the shim, we can't do SRI! While I still don't love the idea of checking all JS deps into the project repo, I do appreciate your ongoing push to make the JavaScript story in Rails simpler with less moving parts. Onward!