make directory/churn.txt download origin configurable

[PascalinDe]

Currently, both the latest directory cache and churn.txt files are downloaded from our GitHub releases w/ a hard-coded link:

lightarti-rest-android/artiwrapper/src/main/java/org/c4dt/artiwrapper/TorLibApi.java

Line 40 in f5bde33

public static final String DIRECTORY_CACHE_C4DT = "https://github.com/c4dt/lightarti-directory/releases/latest/download/directory-cache.tgz";

This means that someone could not host these files themselves if they'd prefer so. These variables should be retrieved from a configuration file/the environment to allow for this.

[ineiti]

Or even better: move this code to lightarti-rest, so it doesn't have to be implemented in the iOS library, too. Because currently on iOS it just takes the files in the directory of the library :(