Support export for the BeyondTheRepository BagIt profile

[mikejritter]

Partially resolves (imo): https://jira.lyrasis.org/browse/FCREPO-3176

This adds support for some of the newer additions in both the bagit-profiles-spec and the beyond the repository bagit profile:

• BagIt-Profile-Identifier is now written to the bag-info.txt
  • BagIt-Profile-Version is not, but it looks it will be required for validation of v1.2.0 (though currently this passes validation using the bagit-profiles-validator)
• Bag-Info now supports the repeatable and description options
  • recommended was also added from the beyond the repository spec; a warning is logged on any missing recommended fields
  • currently it is not possible to have repeated values in the bagit configs, so it is there only for completion
• Tag-Files-Allowed is checked against when validating a BagConfig

I also opted to read in the full bag profile json in BagProfile in anticipation of doing more validation against BagIt Bags, as I think there's some additional opportunity for both the BagIt Profiles and import of BagIt Bags.

For the BagIt Profiles, you'll find requirements mentioned throughout the spec such as:

Every file in Tag-Files-Required must also be present in Tag-Files-Allowed.

This I haven't added yet but could be done either when reading the BagIt Profile json or with the BagConfig validation.

For the Import, it looks like work had been started for validating BagIt Bags before running the Import process itself but had been put on hold. I don't think would be onerous to add support for ensuring a Bag conforms to a profile, but I wanted to keep this PR scoped to export.

[awoods]

@mikejritter : Thanks for this work!

@bbranan : I will be offline the rest of the week. If my comments resolved/addressed before next week and you want to get this merged, feel free. Otherwise, I will approve/merge next week.

[bbranan]

The updates based on comments look good to me. @awoods feel free to merge when you've had a chance to take a look.