GraphQL: Allow filtering epics by top level only

What does this MR do and why?

Adds a new argument topLevelHierarchyOnly to the epics query that allows us to fetch top-level epics only (IOW, epics with no parent).

We are also modifying EpicsFinder to accept the new param top_level_hierarchy_only. This new param will be ignored if the parent_id param is present because we shouldn't be able to request top_level_hierarchy_only when we are trying to find children of parent_id (the result would be empty).

These are the backend changes needed to fix #337435 (closed).

Example query

query topLevelEpics {
  group(fullPath: $group_id) {
    epics(topLevelHierarchyOnly: true) {
      edges {
        node {
          title
          hasParent
        }
      }
    }
  }
}

All results from this query should return "hasParent": false.

Database impact

The finder query will include an additional condition when top_level_hierarchy_only param is set as true: AND "epics"."parent_id" IS NULL.

With top_level_hierarchy_only: true

EpicsFinder.new(user, { group_id: 9970, top_level_hierarchy_only: true}).execute

SQL

SELECT
  "epics".*
FROM
  "epics"
WHERE
  "epics"."group_id" IN (
    SELECT
      "namespaces"."id"
    FROM
      "namespaces"
    WHERE
      "namespaces"."type" = 'Group'
      AND (traversal_ids @> ('{9970}'))
  )
  AND "epics"."parent_id" IS NULL
ORDER BY
  "epics"."id" DESC
LIMIT
  100

Query plan (internal)

With top_level_hierarchy_only: false

EpicsFinder.new(user, { group_id: 9970, top_level_hierarchy_only: false}).execute

SQL

SELECT
  "epics".*
FROM
  "epics"
WHERE
  "epics"."group_id" IN (
    SELECT
      "namespaces"."id"
    FROM
      "namespaces"
    WHERE
      "namespaces"."type" = 'Group'
      AND (traversal_ids @> ('{9970}'))
  )
ORDER BY
  "epics"."id" DESC
LIMIT
  100

Query plan (internal)

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