Add support for checksum string generation

Some times it's necessary to verify if a given file is equaled to another file inside hdfs, it could happen when we are trying to verify if the file content is different from an older version.

For that, we have to implement a method to get the string checksum from a file inside hdfs and return the checksum as a String to be used when needed.

For now, this issue doesn't relate to the generation of a cross filesystem checksum.