Add graphql for workflow artifacts

What does this MR do and why?

Adds GraphQL functionality for workflow "linkables". Links between a workflow work_items for now. Followups will add merge requests, notes, and pipelines.

References

Closes #604024 (closed)

How to set up and validate locally

1. Set up Duo Agent Platform
2. Create links between a workflow and work item. You can go to an issue and ask developer to create a new issue. This should created two links between the workflow and the source work item and the created work item.
3. Test out the below graphql queries:

Query 1: Get all workflows that were created FROM a work item:

query WorkflowsfromSource($workItemId: WorkItemID!) {                                                                                                                                        
  workItem(id: $workItemId) {                                                                                                                                                                       
    id                                                                                                                                                                                              
    duoWorkflowLinks(linkType: SOURCE) {                                                                                                                                                                              
      nodes {                                                                                                                                                                                       
        linkType                                                                                                                                                                                    
        workflow {                                                                                                                                                                                  
          id                                                                                                                                                                                        
	  title                                                                                                                                                                          
        }                                                                                                                                                                                           
      }                                                                                                                                                                                             
    }                                                                                                                                                                                               
  }                                                                                                                                                                                                 
}    

Query 2: Get all work items linked to a workflow:

query workflowLinkables {
  duoWorkflowWorkflows(workflowId: "gid://gitlab/Ai::DuoWorkflows::Workflow/4561") {
    nodes {
      id
      title
      workItemLinks {                                                                                                                                                                           
        nodes {                                                                                                                                                                                 
          linkType                                                                                                                                                                              
          workItem { id title }                                                                                                                                                                 
        }                                                                                                                                                                                       
      }    
    }
  }
} 

New scope

Here is the DB query when using the new scope:

SELECT "duo_workflows_workflow_work_items".* FROM "duo_workflows_workflow_work_items" WHERE "duo_workflows_workflow_work_items"."workflow_id" = 5304878 AND "duo_workflows_workflow_work_items"."link_type" = 0

Execution Plan:

 Index Scan using index_duo_wf_wf_wi_on_workflow_id_and_work_item_id on public.duo_workflows_workflow_work_items  (cost=0.28..3.30 rows=1 width=58) (actual time=1.798..1.799 rows=0 loops=1)
   Index Cond: ((duo_workflows_workflow_work_items.workflow_id = 5304878) AND (duo_workflows_workflow_work_items.link_type = 0))
   Buffers: shared hit=3 read=2
Settings: effective_cache_size = '472585MB', jit = 'off', random_page_cost = '1.5', work_mem = '230MB', seq_page_cost = '4'

DBLab Link with execution plan

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