Add zucchini support in update_engine

Support zucchini as a diff algorithm in delta generator.
Note we won't update the max support version until the
installation support is in place.

Bug: 197361113
Test: TH, generate a diff package
Change-Id: I92db1e7ab2ac4aef104a715c089b136f138064e9
diff --git a/update_metadata.proto b/update_metadata.proto
index 533f6f0..6164a4d 100644
--- a/update_metadata.proto
+++ b/update_metadata.proto
@@ -187,6 +187,9 @@
 
     // On minor version 5 or newer, these operations are supported:
     PUFFDIFF = 9;  // The data is in puffdiff format.
+
+    // On minor version 8 or newer, these operations are supported:
+    ZUCCHINI = 11;
   }
   required Type type = 1;