Adds support to exclude fields from _source

[JPrevost]

Why are these changes being introduced:

• We are starting to add very large fields into our OpenSearch _source, which we are concerned may cause performance issues

Relevant ticket(s):

• https://mitlibraries.atlassian.net/browse/USE-406

How does this address that need:

• Introduces an ENV variable to specify fields to exclude from _source

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

[jazairi]

I haven't verified locally, but the code is clear and the test coverage gives me confidence that this works as expected.