Add rake task for running checksum validation on a single druid

[andrewjbtw]

For certain remediation tasks, I run checksum validation on individual druids so that I can be sure that everything is in a good state. The command I run is listed in the wiki:

Audit::ChecksumValidatorUtils.validate_druid(druid)

After the recent preservation maintenance, this command was moved out of the readme as part of a broader documentation simplification effort. It's now on a page that is marked as not being maintained. The command still works but the code could change in the future.

After a brief Slack conversation, the suggestion was that if this task is made into a rake task, then it will be less likely to be left out of future development because it will be in the code, rather than in an old wiki page.