Display specific fields in the index and metadata configuration page only for Parker exhibit

[hudajkhan]

Relates to #2622 .

What this PR does:

• Create a new feature flag for parker specific index fields
• Sets that flag to false in the main settings file.
• In the catalog controller, only adds parker specific fields if that flag is set to true.

How to test this:

• In local development, create an exhibit with the slug "parker".
• Set up Solr. Add a single Solr document with the following fields indicating this is a parker exhibit:

("exhibit_parker_public_bsi": [
    true
  ],
  "spotlight_exhibit_slug_parker_bsi": [
    true
  ],
  "spotlight_exhibit_slugs_ssim": [
    "parker"
  ]

• create a new settings.local.yml file (or similar).
  Set up:
  feature_flags:
  parker:
  add_parker_index_fields: true

You should still see Parker specific fields in the metadata configuration page as well as the actual search results view, and you should not see Parker specific fields in any exhibit that is not Parker.

I didn't see any specific feature flag tests around values i.e. add_parker_search_fields. If anyone has thoughts on any tests to add, please feel free to suggest them.

Next step once this is merged: Updating feature flags for parker and parker-test here https://github.com/sul-dlss/shared_configs/blob/exhibits-prod/config/settings/production.yml so that the right values are picked up