BE: Add new query API to list available agents under a group in RD

Issue: #441555 (closed)

What does this MR do and why?

• Adds query API to return the list of list remote development agents during workspace creation
• The list of agents returned are based on the agents that may be mapped to the namespace being passed or its ancestor namespaces
• More details can be found in the relevant section within the technical design

MR acceptance checklist

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Query Plan

For the new scope:

gitlabhq_development=# explain select * from remote_development_namespace_cluster_agent_mappings where namespace_id in (21, 22, 24);
                                                                                 QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using unique_namespace_cluster_agent_mappings_for_agent_association on remote_development_namespace_cluster_agent_mappings  (cost=0.15..11.73 rows=16 width=48)
   Index Cond: (namespace_id = ANY ('{21,22,24}'::bigint[]))
(2 rows)

For the filtering of valid mappings:

gitlabhq_development=# explain select * from cluster_agents where id in (2);
                                        QUERY PLAN
-------------------------------------------------------------------------------------------
 Index Scan using cluster_agents_pkey on cluster_agents  (cost=0.15..2.17 rows=1 width=73)
   Index Cond: (id = 2)
(2 rows)

gitlabhq_development=# explain select * from projects where id in (3);
                                                           QUERY PLAN                                                         
---------------------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_projects_on_repository_storage_last_repository_updated_at on projects  (cost=0.14..2.16 rows=1 width=1443)
   Index Cond: (id = 3)
(2 rows)

gitlabhq_development=# explain select * from namespaces where id in (26);
                                     QUERY PLAN
------------------------------------------------------------------------------------
 Index Scan using namespaces_pkey on namespaces  (cost=0.14..2.16 rows=1 width=533)
   Index Cond: (id = 26)
(2 rows)

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Before	After

How to set up and validate locally

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