Draft: 馃憜 add repo fingerprint to index v2

Currently there's no way to query a F-Droid repository for it's fingerprint. (I'm particularly referring to the fingerprint used as query-parameter in repo urls.) I gets published to the generated repository index website, but parsing html isn't really better than trying to parse it from singed index jar files. This change adds a that fingerprint as a value to the repo section in index-v2

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