Zoekt: Add a new setting zoekt_index_all_namespaces

What does this MR do and why?

Why: Currently, the self-managed customers have to manually create ZoektEnabledNamespace from the rails console. With the help of this option, it would be easier for them to get their namespaces indexed.

What: Add a new setting Automatically index root namespaces under the Exact code search configuration. Every 10 minutes our rake task will check if the setting zoekt_auto_index_root_namespace is enabled and if there is any root group that doesn't have zoekt_enabled_namespace, it will create Search::Zoekt::EnabledNamespace for that root group. We don't need to run this task on gitlab.com.

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Before	After

How to set up and validate locally

• Run the gdk and navigate to the http://127.0.0.1:3000/admin/application_settings/advanced_search
• Expand the Exact code search configuration.
• Uncheck the checkbox Automatically index root namespaces
• Run the rails console

bin/rails c

• Check if you have a root group which has missing zoekt_enabled_namespace. If not then create a root group from the browser and check it again.

Group.root_groups_without_zoekt_enabled_namespace.any?

• Check the Search::Zoekt::EnabledNamespace count

Search::Zoekt::EnabledNamespace.count

• Run the task

Search::Zoekt::SchedulingService.new(:create_zoekt_indexed_namespace_for_self_managed).execute

• Check the Search::Zoekt::EnabledNamespace count. It should not be changed

Search::Zoekt::EnabledNamespace.count

• Now check the checkbox Automatically index root namespaces under the application setting.
• You need to restart the rails console to take effect the Application setting change.
• Run the task again. And check the count of Search::Zoekt::EnabledNamespace. It should have been increased.
• Check the count of Group.root_groups_without_zoekt_enabled_namespace. Should be 0

Query plan

SELECT
    "namespaces"."id"
FROM
    "namespaces"
    LEFT OUTER JOIN "zoekt_enabled_namespaces" "zoekt_enabled_namespace" ON "zoekt_enabled_namespace"."root_namespace_id" = "namespaces"."id"
WHERE
    "namespaces"."type" = 'Group'
    AND "namespaces"."parent_id" IS NULL
    AND "zoekt_enabled_namespace"."root_namespace_id" IS NULL
LIMIT 1000

https://console.postgres.ai/shared/00eaba4f-5367-4fd5-bbee-789af7c0a36d

Related to #455533 (closed)