Add keyset pagination support for /users endpoint

What does this MR do and why?

It introduces keyset pagination for /users endpoint. This should solve the performance problem we have with the endpoint (see SQL section below).

As I wanted to enable keyset pagination for multiple orderings, I changed the CursorBasedKeyset class to support this. In order to avoid potential problems, the changes are under a feature flag. (follow-up issue for rollout here).

SQL

• Query with offset pagination: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/21640/commands/70306
• Query with keyset pagination: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/21640/commands/70309

With keyset pagination ordered by:

• by id
• by username
• by name
• by created_at
• by updated_at

How to set up and validate locally

1. In rails console enable feature flag

   Feature.enable(:api_keyset_pagination_multi_order)

2. Visit the changed endpoint with offset pagination (old)

   curl -i --header "Authorization: Bearer TOKEN" "https://gdk.test:3443/api/v4/users?per_page=10&page=1"

3. Check the headers include offset pagination headers (eg. url includes page param, headers include X-Next-Page, X-Page, or X-Total-Pages`

4. Visit the changed endpoint with keyset pagination (new)

   curl -i --header "Authorization: Bearer TOKEN" "https://gdk.test:3443/api/v4/users?pagination=keyset&per_page=10&page=1"

5. Check the headers include url in headers, url should include cursor. The headers should not include X-Next-Page, X-Page, or X-Total-Pages`

6. Check with other orderings we support for the endpoint (id, name, username), eg.

   curl -i --header "Authorization: Bearer TOKEN" "https://gdk.test:3443/api/v4/users?pagination=keyset&order_by=username&sort=asc&per_page=10&page=1"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #419556 (closed)