S3BinaryCacheStore: Support compression of narinfo and log files

You can now set the store parameter "text-compression=br" to compress
textual files in the binary cache (i.e. narinfo and logs) using
Brotli. This sets the Content-Encoding header; the extension of
compressed files is unchanged.

You can separately specify the compression of log files using
"log-compression=br". This is useful when you don't want to compress
narinfo files for backward compatibility.

src/libstore/download.hh

@@ -73,4 +73,7 @@ public:
 
 bool isUri(const string & s);
 
+/* Decode data according to the Content-Encoding header. */
+ref<std::string> decodeContent(const std::string & encoding, ref<std::string> data);
+
 }