Make node_id optional for Zoekt Admin API to add/remove namespace

What does this MR do and why?

We want to use create zoekt_indexed_namespace without providing the node_id. So, allowing this API to accept 0 as node_id. On the index creation endpoint, it will skip creating the Search::Zoekt::Index record when the node_id is 0. It will only create the record for Search::Zoekt::EnabledNamespace. In the case of DELETE when node_id is 0 it will remove the Search::Zoekt::Index records from all the nodes

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Before	After

How to set up and validate locally

Testing for PUT endpoint

• Create a new root namespace in the browser
• Open a rails console

bin/rails c

• Check the count of Search::Zoekt::EnabledNamespace and Search::Zoekt::Index
• Now run the following the curl command. Replace access_token with your personal access token and namespace_id with the one you have created at the first step.

curl -X PUT --location 'http://127.0.0.1:3000/api/v4/admin/zoekt/shards/0/indexed_namespaces/:namespace_id?access_token=#{access_token}'

• Now verify again the counts of Search::Zoekt::EnabledNamespace and Search::Zoekt::Index. The count of Search::Zoekt::EnabledNamespace should be incremented by 1 but the count of Search::Zoekt::Index should remain the same.

Testing for DELETE endpoint

• Create a new Zoekt node

node = Search::Zoekt::Node.last
node2 = Search::Zoekt::Node.create! index_base_url: node.index_base_url, search_base_url: node.search_base_url, uuid: SecureRandom.uuid

• Create indices for a Search::Zoekt::EnabledNamespace

zoekt_enabled_namespace = Search::Zoekt::EnabledNamespace.last
Search::Zoekt::Index.create! zoekt_enabled_namespace_id: zoekt_enabled_namespace.id, zoekt_node_id: Search::Zoekt::Node.first.id, namespace_id: zoekt_enabled_namespace.namespace.id
Search::Zoekt::Index.create! zoekt_enabled_namespace_id: zoekt_enabled_namespace.id, zoekt_node_id: Search::Zoekt::Node.last.id, namespace_id: zoekt_enabled_namespace.namespace.id

• Verify that the following count is 2

zoekt_enabled_namespace.indices.count

• Now run the following curl command. Replace access_token with your personal access token and namespace_id with the one you have created at the first step.

curl -X DELETE --location 'http://127.0.0.1:3000/api/v4/admin/zoekt/shards/0/indexed_namespaces/:namespace_id?access_token=#{access_token}'

• Now verify that the following counts are 0. Replace namespace_id with the one you created

Search::Zoekt::EnabledNamespace.where(root_namespace_id: "#{namespace_id}").count
Search::Zoekt::Index.where(namespace_id: "#{namespace_id}").count

Related to #436498 (closed)