Add new index library

This add a new F-Droid multi-platform index library that can parse V1 and V2 index, directly into memory or via a stream receiver.

Note that some features are only available for Android:

• index signature verification (JarFile is JVM only)
• index stream processing (InputStream is JVM only)
• index V2 diffing (reflection is JVM only)
• app device compatibility checking (requires Android)
• app update checking (requires Android)

Other platforms besides Android have not been tested and might need additional work.