Construct DSS JSON

[jazairi]

Why these changes are being introduced:

We want to be able to publish theses to DSpace, which requires
us to convert relevant thesis metadata to DSS-compliant JSON.

Relevant ticket(s):

• https://mitlibraries.atlassian.net/browse/ETD-394

How this addresses that need:

This creates the requisite submission metadata for DSS, based on the
submission message spec and the data mappings spreadsheet. It does not
save said metadata to AWS, nor does it write messages to the queue.
I'm open to including that work in the same PR, but this seeemed like
a logical breakpoint.

Side effects of this change:

There are fields that are required for publication that are not
validated anywhere in the application. The current process is for
Jess to confirm these as part of their workflow, but we should make
their life easier by including an automated check as part of
the publication status update workflow. A ticket for this work is
open here: https://mitlibraries.atlassian.net/browse/ETD-431

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

NO

[coveralls]

Coverage increased (+0.2%) to 95.087% when pulling 4bd4a37 on etd-394-construct-dss-json into 22f65ee on main.