Add bag validation service

[zflamig]

Determine if the bdbag is a valid structure and optionally if it contains valid data.

[NickolausDS]

Could you provide a little more context around this issue?

I have had a conversation like this with Mike Darcy on BDBag metadata, and we reached the conclusion that it would be better to have BDBag validate BDBag specific data and have the Concierge service simply pass along any errors if there are any.