Filter: Added window_bits field to gzip proto (#356)

Signed-off-by: Gabriel <gsagula@gmail.com>

api/filter/http/gzip.proto

@@ -74,4 +74,13 @@ message Gzip {
   // Allows disabling compression if response contains "last-modified"
   // header. Default is false.
   google.protobuf.BoolValue disable_on_last_modified = 8;
+
+  // Value from 9 to 15 that represents the base two logarithm of the compressor's window size.
+  // Larger values result in better compression at the expense of memory usage; e.g. 12 will produce
+  // a 4096 bytes window. Default is 15. For more details about this parameter, please refer to Zlib
+  // manual > deflateInit2.
+  // Note that due to a known bug in the underlying zlib library, window bits
+  // with value 8 does not work as expected, therefore 9 is the smallest window size supported by
+  // this filter at the moment. This issue might be solved in future releases of the library.
+  google.protobuf.UInt32Value window_bits = 9 [(validate.rules).uint32 = {gte: 9, lte: 15}];
 }