Add Import Source Users to GraphQL

What does this MR do and why?

Add resolver to return all import source users associated with a namespace

Add mutations to manage import source users

• Cancel reassignment
• Reassign
• Keep as placeholder

Related to: #457066

SQL queries

Raw SQL

SELECT
    "import_source_users".*
FROM
    "import_source_users"
WHERE
    "import_source_users"."namespace_id" = 1
ORDER BY
    "import_source_users"."id" DESC
LIMIT 101

Query plan

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29382/commands/91420

 Limit  (cost=0.01..0.02 rows=1 width=218) (actual time=0.043..0.043 rows=0 loops=1)
   Buffers: shared hit=3
   I/O Timings: read=0.000 write=0.000
   ->  Sort  (cost=0.01..0.02 rows=1 width=218) (actual time=0.041..0.042 rows=0 loops=1)
         Sort Key: import_source_users.id DESC
         Sort Method: quicksort  Memory: 25kB
         Buffers: shared hit=3
         I/O Timings: read=0.000 write=0.000
         ->  Seq Scan on public.import_source_users  (cost=0.00..0.00 rows=1 width=218) (actual time=0.008..0.009 rows=0 loops=1)
               Filter: (import_source_users.namespace_id = 9970)
               Rows Removed by Filter: 0
               I/O Timings: read=0.000 write=0.000

Time: 0.995 ms  
  - planning: 0.880 ms  
  - execution: 0.115 ms  
    - I/O read: 0.000 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 3 (~24.00 KiB) from the buffer pool  
  - reads: 0 from the OS file cache, including disk I/O  
  - dirtied: 0  
  - writes: 0  

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