feat(datastore): add index on manifests subject_id field

What does this MR do?

⚠️ Marking as draft to prevent merging this before the %17.10 cut off date.

In preparation for !2086 (merged), this adds the required indexes to make the new GC query performant. We need to merge and release this change first.

Related to #1489 (closed)

Query plan for !2086 (merged) after creating this index:

localhost jdrpereira@gitlabhq_registry_dblab=# explain (analyze) SELECT 1 FROM tags
    WHERE top_level_namespace_id = 1569
      AND repository_id = 1632081
      AND manifest_id = 18534278
    UNION ALL
    SELECT 1 FROM manifest_references
    WHERE top_level_namespace_id = 1569
      AND repository_id = 1632081
      AND child_id = 18534278
    UNION ALL
    SELECT 1 FROM manifests
    WHERE top_level_namespace_id = 1569
      AND repository_id = 1632081
      AND subject_id = 18534278;
Append  (cost=0.43..10.21 rows=3 width=4) (actual time=7.142..9.360 rows=1 loops=1)
  ->  Index Only Scan using index_tags_p_27_on_ns_id_and_repo_id_and_manifest_id_and_name on tags_p_27 tags  (cost=0.43..3.45 rows=1 width=4) (actual time=7.141..7.142 rows=1 loops=1)
        Index Cond: ((top_level_namespace_id = 1569) AND (repository_id = 1632081) AND (manifest_id = 18534278))
        Heap Fetches: 0
  ->  Index Only Scan using manifest_references_p_27_top_level_namespace_id_repository__idx on manifest_references_p_27 manifest_references  (cost=0.29..3.31 rows=1 width=4) (actual time=0.935..0.935 rows=0 loops=1)
        Index Cond: ((top_level_namespace_id = 1569) AND (repository_id = 1632081) AND (child_id = 18534278))
        Heap Fetches: 0
  ->  Index Only Scan using index_manifests_p_27_on_ns_id_and_repo_id_and_subject_id on manifests_p_27 manifests  (cost=0.43..3.44 rows=1 width=4) (actual time=1.280..1.280 rows=0 loops=1)
        Index Cond: ((top_level_namespace_id = 1569) AND (repository_id = 1632081) AND (subject_id = 18534278))
        Heap Fetches: 0
Planning Time: 69.209 ms
Execution Time: 9.447 ms
Time: 275.770 ms

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