Draft: Add index on members source and role

What does this MR do and why?

Before:

SELECT "members"."access_level", COUNT(*) AS members_count
FROM
  (
    (
      SELECT
        "members"."id",
        "members"."access_level",
        "members"."source_id",
        "members"."source_type",
        "members"."user_id",
        "members"."notification_level",
        "members"."type",
        "members"."created_at",
        "members"."updated_at",
        "members"."created_by_id",
        "members"."invite_email",
        "members"."invite_token",
        "members"."invite_accepted_at",
        "members"."requested_at",
        "members"."expires_at",
        "members"."ldap",
        "members"."override",
        "members"."state",
        "members"."invite_email_success",
        "members"."member_namespace_id",
        "members"."member_role_id",
        "members"."expiry_notified_at",
        "members"."request_accepted_at"
      FROM "members"
      WHERE "members"."source_type" = 'Namespace'
        AND "members"."source_id" IN (
          SELECT "namespaces"."id"
          FROM "namespaces"
          WHERE "namespaces"."type" = 'Group'
            AND (traversal_ids @> ('{70960922}'))
        )
    )
    UNION
    (
      SELECT
        "members"."id",
        "members"."access_level",
        "members"."source_id",
        "members"."source_type",
        "members"."user_id",
        "members"."notification_level",
        "members"."type",
        "members"."created_at",
        "members"."updated_at",
        "members"."created_by_id",
        "members"."invite_email",
        "members"."invite_token",
        "members"."invite_accepted_at",
        "members"."requested_at",
        "members"."expires_at",
        "members"."ldap",
        "members"."override",
        "members"."state",
        "members"."invite_email_success",
        "members"."member_namespace_id",
        "members"."member_role_id",
        "members"."expiry_notified_at",
        "members"."request_accepted_at"
      FROM "members"
      WHERE "members"."source_type" = 'Project'
        AND "members"."source_id" IN (
          SELECT "projects"."id"
          FROM "projects"
          WHERE "projects"."namespace_id" IN (
              SELECT namespaces.traversal_ids [array_length(namespaces.traversal_ids, 1)] AS id
              FROM "namespaces"
              WHERE "namespaces"."type" = 'Group'
                AND (traversal_ids @> ('{70960922}'))
            )
        )
    )
  ) members
WHERE "members"."member_role_id" IS NULL
GROUP BY "members"."access_level"

Time: 6.412 min
  - planning: 6.549 ms
  - execution: 6.412 min
    - I/O read: 6.103 min
    - I/O write: 138.583 ms

Shared buffers:
  - hits: 989059 (~7.50 GiB) from the buffer pool
  - reads: 1405254 (~10.70 GiB) from the OS file cache, including disk I/O
  - dirtied: 81064 (~633.30 MiB)
  - writes: 5242 (~41.00 MiB)

Temp buffers:
  - reads: 56709 (~443.00 MiB)
  - writes: 56716 (~443.10 MiB)

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/33194/commands/102515

After:

SELECT "members"."access_level", COUNT(*) AS members_count
FROM
  (
    (
      SELECT
        "members"."id",
        "members"."access_level",
        "members"."source_id",
        "members"."source_type",
        "members"."user_id",
        "members"."notification_level",
        "members"."type",
        "members"."created_at",
        "members"."updated_at",
        "members"."created_by_id",
        "members"."invite_email",
        "members"."invite_token",
        "members"."invite_accepted_at",
        "members"."requested_at",
        "members"."expires_at",
        "members"."ldap",
        "members"."override",
        "members"."state",
        "members"."invite_email_success",
        "members"."member_namespace_id",
        "members"."member_role_id",
        "members"."expiry_notified_at",
        "members"."request_accepted_at"
      FROM "members"
      WHERE "members"."source_type" = 'Namespace'
        AND "members"."source_id" IN (
          SELECT "namespaces"."id"
          FROM "namespaces"
          WHERE "namespaces"."type" = 'Group'
            AND (traversal_ids @> ('{70960922}'))
        )
    )
    UNION
    (
      SELECT
        "members"."id",
        "members"."access_level",
        "members"."source_id",
        "members"."source_type",
        "members"."user_id",
        "members"."notification_level",
        "members"."type",
        "members"."created_at",
        "members"."updated_at",
        "members"."created_by_id",
        "members"."invite_email",
        "members"."invite_token",
        "members"."invite_accepted_at",
        "members"."requested_at",
        "members"."expires_at",
        "members"."ldap",
        "members"."override",
        "members"."state",
        "members"."invite_email_success",
        "members"."member_namespace_id",
        "members"."member_role_id",
        "members"."expiry_notified_at",
        "members"."request_accepted_at"
      FROM "members"
      WHERE "members"."source_type" = 'Project'
        AND "members"."source_id" IN (
          SELECT "projects"."id"
          FROM "projects"
          WHERE "projects"."namespace_id" IN (
              SELECT namespaces.traversal_ids [array_length(namespaces.traversal_ids, 1)] AS id
              FROM "namespaces"
              WHERE "namespaces"."type" = 'Group'
                AND (traversal_ids @> ('{70960922}'))
            )
        )
    )
  ) members
WHERE "members"."member_role_id" IS NULL
GROUP BY "members"."access_level"

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