Move helper methods and enums into a common util file

Summary:
Extract helper functions for serializing/deserializing the hash value of aux data. We put this into a utility file so we can reuse these helpers when we need to serialize the Tree aux data so we can store it into the local storage.

## 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: D72340694

fbshipit-source-id: 3351f158872304bb7bb0b929199593414a2f927a

Author: vilatto

eden/common/utils/Hash.h

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <folly/Range.h>
+
+#include "eden/common/utils/Throw.h"
+#include "eden/fs/model/Hash.h"
+
+namespace facebook::eden {
+
+// bit enum representing possible hash types that could be used
+// 8 should be more than enough for now
+// but still this enum is represented as a variant
+enum class HashType : uint8_t {
+  SHA1 = (1 << 0),
+  BLAKE3 = (1 << 1),
+};
+
+FOLLY_ALWAYS_INLINE void
+write(const uint8_t* src, size_t len, uint8_t* dest, size_t& off) {
+  memcpy(dest + off, src, len);
+  off += len;
+}
+
+/**
+ * Read the hash from a byte range to the destination Hash buffer.
+ *
+ * This is currently used by BlobAuxData and TreeAuxData.
+ */
+template <size_t SIZE>
+void readAuxDataHash(
+    const ObjectId& id,
+    folly::ByteRange& bytes,
+    Hash<SIZE>& hash) {
+  if (bytes.size() < SIZE) {
+    throwf<std::invalid_argument>(
+        "auxData for {} had unexpected size {}. Could not deserialize the hash of size {}.",
+        id,
+        bytes.size(),
+        SIZE);
+  }
+
+  auto mutableBytes = hash.mutableBytes();
+  memcpy(mutableBytes.data(), bytes.data(), mutableBytes.size());
+  bytes.advance(mutableBytes.size());
+}
+
+} // namespace facebook::eden