:: DeveloperApi ::
Identifies a particular Block of data, usually associated with a single file.
A Block can be uniquely identified by its filename, but each type of Block has a different
set of keys which produce its unique name.
If your BlockId should be serializable, be sure to add it to the BlockId.apply() method.
A globally unique identifier for this Block.
A globally unique identifier for this Block. Can be used for ser/de.