Add fsverity_metadata_generator helper binary

Making this a host tool will help users generate their own fsverity
metadata easily.

Bug: 205987437
Test: m fsverity_metadata_generator and run it
Change-Id: Iafd228815a74d298d87ca1466c6909c0d24c5874
diff --git a/tools/releasetools/Android.bp b/tools/releasetools/Android.bp
index a979a8e..bf7f9a0 100644
--- a/tools/releasetools/Android.bp
+++ b/tools/releasetools/Android.bp
@@ -553,6 +553,19 @@
     ],
 }
 
+python_binary_host {
+    name: "fsverity_metadata_generator",
+    srcs: [
+        "fsverity_metadata_generator.py",
+    ],
+    libs: [
+        "fsverity_digests_proto_python",
+    ],
+    required: [
+        "fsverity",
+    ],
+}
+
 //
 // Tests.
 //