declare physical/legal mirror location in index

Mirrors are almost always very statically placed servers, especially official mirrors. The index should support metadata for each declared mirror that describes where it is physically/legally located. Then the client(s) can make a decision which mirror would work best based only on data it already has locally. E.g if the user sets the localization country to AT, then fdroidclient could choose to only use mirrors in Austria (AT) or only use mirrors outside of Austria.