Further fixes for unicode handling in manifests

We were occasionally trying to compute schema 2 version 1 signatures on the *unicode* representation, which was failing the signature check. This PR adds a new wrapper type called `Bytes`, which all manifests must take in, and which handles the unicodes vs encoded utf-8 stuff in a central location. This PR also adds a test for the manifest that was breaking in production.

endpoints/api/manifest.py

@@ -72,7 +72,7 @@ def _manifest_dict(manifest):
   return {
     'digest': manifest.digest,
     'is_manifest_list': manifest.is_manifest_list,
-    'manifest_data': manifest.manifest_bytes,
+    'manifest_data': manifest.internal_manifest_bytes.as_unicode(),
     'image': image,
     'layers': ([_layer_dict(lyr.layer_info, idx) for idx, lyr in enumerate(layers)]
                if layers else None),