CMR-9859: Adding variable association to the generic association endpoint. Allow variables to be ingested without a collection association.

[eereiter]

Overview

What is the feature/fix?

Adding variable association to the generic association endpoint. Allow variables to be ingested without a collection association.

What is the Solution?

Adding variable association to the generic association endpoint. Allow variables to be ingested without a collection association.

What areas of the application does this impact?

Ingest, Search, Metadata_db.

Checklist

• [x] I have updated/added unit and integration tests that prove my fix is effective or that my feature works
• [x] New and existing unit and int tests pass locally and remotely with my changes
• [x] I have removed unnecessary/dead code and imports in files I have changed
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have cleaned up integration tests by doing one or more of the following:
  • de-duped, consolidated, removed dead int tests
  • transformed applicable int tests into unit tests
  • refactored to reduce number of system state resets by updating fixtures (use-fixtures :each (ingest/reset-fixture {})) to be :once instead of :each

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.38674% with 126 lines in your changes are missing coverage. Please review.

Project coverage is 57.98%. Comparing base (c8c9da8) to head (79dbc93).

Files	Patch %	Lines
...p/src/cmr/metadata_db/services/concept_service.clj	30.76%	51 Missing and 3 partials :warning:
...ngest-app/src/cmr/ingest/api/generic_documents.clj	20.00%	32 Missing :warning:
...pp/src/cmr/search/services/association_service.clj	5.55%	17 Missing :warning:
...src/cmr/search/services/association_validation.clj	47.82%	12 Missing :warning:
...mr/search/services/generic_association_service.clj	0.00%	11 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2123 +/- ## ========================================== + Coverage 57.77% 57.98% +0.20% ========================================== Files 1041 1041 Lines 70286 70151 -135 Branches 1991 1994 +3 ========================================== + Hits 40609 40675 +66 + Misses 27828 27617 -211 - Partials 1849 1859 +10 ```

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