Include Tree aux data in the Tree response from sapling backing store

Summary:
Modify `fromRawTree` so it can populate the tree aux data when converting from a sapling tree into the eden tree. We check the returned digest_size of the tree aux data to determine if it's a valid aux data. We skip storing it if it's invalid.
Later we might want to force sapling to always return a valid tree aux data but for now let's go with the naive way.

## Context
Currently we fetch TreeAuxData directly from the backing store without caching. We want to add cache support. The cache will be consist of the in-memory cache (in ObjectStore) as well as the local store.

Reviewed By: jdelliot

Differential Revision: D73795248

fbshipit-source-id: a8524760a90780842b73c036df4218bcaaf030d5

Author: vilatto

third-party/watchman/src/eden/fs/service/eden.thrift

@@ -974,11 +974,18 @@ union ScmTreeOrError {
   2: EdenError error;
 }
 
+struct TreeAux {
+  1: DigestSizeOrError digestSize;
+  2: DigestHashOrError digestHash;
+}
+
 struct ScmTreeWithOrigin {
   # the SCM tree data
   1: ScmTreeOrError scmTreeData;
   # where the SCM tree was fetched from
   2: DataFetchOrigin origin;
+  # include the Tree Aux when it's available
+  3: optional TreeAux treeAux;
 }
 
 struct DebugGetScmTreeResponse {