This adds support for the index fields: icon, mirrors, and antiFeatures.
icon and mirrors are for Repo, they've been around a while on the server
side, but just never used on the client side.
For Apk, this adds a new per-APK antiFeatures field so that each APK can
be individually marked. For example, when tracking is added or removed,
vulnerabilities are discovered and fixed, etc.
These fields will be ignored when using the v0 index.xml format, they
will be used by the upcoming index-v1 format: !422
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