Use Webpack manifest to associate asset references per pack

[aduth]

Why: Since our current approach to list assets required per pack is not scalable, and does not account for packs other than document-capture, nor would it support assets deeply referenced via shareable packages (e.g. @18f/identity-components).

What:

• Implements a Webpack plugin similar to our localization plugin, which scans for calls to getAssetPath
• Associates those paths with the entrypoint(s) they are included within
• When those entrypoints are printed, the paths are mapped using Rails asset_path, and assigned to a window global
• The front-end calls to getAssetPath pull the mapped path from the window global

Testing Instructions:

Ensure there are no regressions to the use of images in the document capture step, e.g. the "warning" icon after a failed submission, or the ID card animated SVG loading interstitial.

[zachmargolis]

LGTM

[aduth]

I made a minor adjustment in b970eba to avoid the use of a "full" script tag, leveraging an approach similar to what we've done with locale strings for components (example.

[aduth]

I expect I'll need to revisit this in future work, where we'll want to use this for the design system icon sprite, which can't be loaded across domains, so we'll need to have a way to pass host: asset_host like we do in IconComponent:

identity-idp/app/components/icon_component.rb

Line 261 in 5a8af56

asset_path([design_system_asset_path('img/sprite.svg'), '#', icon].join, host: asset_host)

Thinking we might need to have some sort of allowlist, e.g.

Suggested change

	asset_map = assets.index_with { |path| asset_path(path) }
	asset_map = assets.index_with do |path|
	asset_path(path, host: SAME_ORIGIN_ASSETS.include?(path) && asset_host)
	end

(host option uses falsey assignment for defaulting)