Add the ability to control which files in the input root are hardlinked

This change acts as a counter proposal to PR #216. Instead of globally
stating whether files should be hardlinked or not, it adds the ability
to let clients control whether files are aliased.

It achieves this by adding a salt field to FileNode. If two files have
the same salt and all other (non-name) properties match, they SHOULD get
constructed in the form of hardlinks.

build/bazel/remote/execution/v2/remote_execution.proto

@@ -779,6 +779,14 @@ message FileNode {
  // The node properties of the FileNode.
  reserved 5;
  NodeProperties node_properties = 6;
+
+ // Salt to force that files in the input root have distinct inode
+ // numbers, even if all properties match.
+ //
+ // Two files with in the input root SHOULD have the same inode number
+ // iff they have the same digest, is_executable flag, node_properties,
+ // and hardlink_salt.
+ bytes hardlink_salt = 7;
 }
 
 // A `DirectoryNode` represents a child of a