fdroid btlog external binary transparency logger

This complements the binary transparency logging that happens on the
server side !226.  Anyone can set up an efficient tracker of any
F-Droid repo which stores all index files that it sees.  It uses HEAD
requests and ETag checking to be as efficient as possible, so that
this can be automatically run at a frequent pace.

fdroid

@@ -42,6 +42,7 @@ commands = {
     "stats": "Update the stats of the repo",
     "server": "Interact with the repo HTTP server",
     "signindex": "Sign indexes created using update --nosign",
+    "btlog": "Update the binary transparency log for a URL",
 }