Introduce Keyset pagination for `GET /groups` API endpoint for unauthenticated users.

What does this MR do?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/331493

With this MR, the GET /groups endpoint will now error out when using offset pagination when all of the following conditions are met:

• The user is anonymous (ie, user accesses this API without logging in. Such users already only get access to data of public groups (visibility_level = 20))
• The user is requesting for records that falls beyond the count of 50,000 (ie, params[:per_page] * params[:page] > 50000)
• The feature flag keyset_pagination_for_groups_api is enabled.

In such a case, the API will error out with the message: Offset pagination has a maximum allowed offset of 50000 for requests that return objects of type Group. Remaining records can be retrieved using keyset pagination.

This anonymous user will now have the option to get the data of public groups using keyset-pagination using the same endpoint, but with an additional params[:pagination] = keyset param included while making the request.

However, the only ordering supported via the params (order_by and sort) for using this keyset pagination will be name ASC.

If the anonymous user uses any other order_by or sort combination in this endpoint while using keyset pagination, it will error out with Keyset pagination is not yet available for this type of request

The short version is: We do not allow anonymous users to keep going thru /groups API using offset pagination anymore beyond 50,000 groups. They will now be forced to use keyset-pagination for the same with name ASC order.

DB Migration

Up Migration

== 20210901065504 AddIndexOnNameAndIdToPublicGroups: migrating ================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, [:name, :id], {:name=>"index_namespaces_public_groups_name_id", :where=>"type = 'Group' AND visibility_level = 20", :algorithm=>:concurrently})
   -> 0.0136s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- add_index(:namespaces, [:name, :id], {:name=>"index_namespaces_public_groups_name_id", :where=>"type = 'Group' AND visibility_level = 20", :algorithm=>:concurrently})
   -> 0.0173s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20210901065504 AddIndexOnNameAndIdToPublicGroups: migrated (0.0349s) =======

Down Migration

== 20210901065504 AddIndexOnNameAndIdToPublicGroups: reverting ================
-- transaction_open?()
   -> 0.0000s
-- indexes(:namespaces)
   -> 0.0113s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- remove_index(:namespaces, {:algorithm=>:concurrently, :name=>"index_namespaces_public_groups_name_id"})
   -> 0.0047s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20210901065504 AddIndexOnNameAndIdToPublicGroups: reverted (0.0194s) =======

Query plans are added at !68346 (comment 665497635).

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How to setup and validate locally (strongly suggested)

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