webpack: implement overrides for icons and manifests fields for non-main builds

[davidmurdoch]

The webpack build system doesn't support loading in manifest.json overrides from specific build types (mmi, flask, etc). It also doesn't support copying files using the fields in our build.yml's features.assets section.

Notes on build.yml features.assets section:

The reason this hasn't been done yet is because I think we should go about it differently. It seems that we sometimes use manifest.json overrides files for customizing fields, sometimes we use build.yml fields, and sometimes we use environment variables. Each way of customization doesn't always overlap with the other ways, but sometimes it does, I think? It's very confusing.
I'd like to eliminate one of these ways of customization in the primary build system, with the webpack build in mind, and then implement it in webpack.

Notes on build-type manifest.json overrides

Since webpack attempts to find all assets automatically (almost always, some assets are copied the CopyPlugin
: _locales and images), I'd like to make our ManifestPlugin learn to parse the fields in the manifest.json file, find all fields that represent assets that need to be copied, and then emitAsset (webpack plugin lingo) those assets so they can be tracked by webpack internals, and mapped to a final manifest.json file. Icon files like images/icon-512.png would end up being something like assets/icon-512.d55afc3b49c5ff325cc1.png and the manifest.json would be altered to reference it ("512": "assets/icon-512.d55afc3b49c5ff325cc1.png").

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity in the last 90 days. It will be closed in 45 days if there is no further activity. The MetaMask team intends on reviewing this issue before close, and removing the stale label if it is still a bug. We welcome new comments on this issue. We do not intend on closing issues if they report bugs that are still reproducible. Thank you for your contributions.

[hjetpoluru]

Issue still exists and removing the stale label