feat(serving): add Hash to FileInfo protos (#5207)

Support having a hash of each file in the xrefs server used potentially
for patching.

kythe/proto/common.proto

@@ -204,3 +204,22 @@ message Language {
   // Set of analyzer versions used for this language's analysis.
   repeated string analyzer_version = 3;
 }
+
+// A Hash of some data.
+message Hash {
+  // Specifies the hash algorithm, if any.
+  enum HashType {
+    NONE = 0;
+
+    // Use a sha256 hash (git's successor hash --
+    // https://git-scm.com/docs/hash-function-transition/).
+    SHA256 = 1;
+    // Use a sha1 hash (git's legacy blob hash).
+    SHA1 = 2;
+  }
+
+  // The type of hash that was performed.
+  HashType type = 1;
+  // The hash value.
+  bytes value = 2;
+}

kythe/proto/serving.proto

@@ -182,6 +182,10 @@ message FileInfo {
   kythe.proto.common.CorpusPath corpus_path = 1;
 
   string revision = 2;
+
+  // Hash of the file contents; the algorithm of the hash may differ between
+  // corpora based on the repository from which they were extracted.
+  kythe.proto.common.Hash hash = 3;
 }
 
 // FileDecorations stores a file's contents and all contained anchor edges.