Add GIN index to member_roles.permissions

What does this MR do and why?

This change adds a GIN index to the member_roles.permissions JSONB column to optimize the queries in !161384 (closed), !161385 (merged) and https://gitlab.com/gitlab-org/gitlab/-/blob/ee92b24be36fb097ce275fff4ace5ff7d75278f9/ee/app/models/preloaders/user_member_roles_in_groups_preloader.rb#L65.

Related to:

• #442851 (closed)
• #477585

MR acceptance checklist

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Screenshots or screen recordings

SELECT "member_roles".* FROM "member_roles" WHERE (member_roles.permissions @> ('{"admin_runners":true}')::jsonb);

Before:

 Seq Scan on public.member_roles  (cost=0.00..502.01 rows=1 width=372) (actual time=10.876..14.663 rows=1 loops=1)
   Filter: (member_roles.permissions @> '{"admin_runners": true}'::jsonb)
   Rows Removed by Filter: 1868
   Buffers: shared read=119 dirtied=2
   I/O Timings: read=13.386 write=0.000

Time: 15.713 ms
  - planning: 0.994 ms
  - execution: 14.719 ms
    - I/O read: 13.386 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 0 from the buffer pool
  - reads: 119 (~952.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 2 (~16.00 KiB)
  - writes: 0

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/30608/commands/94925

After:

 Bitmap Heap Scan on public.member_roles  (cost=4.50..6.01 rows=1 width=372) (actual time=0.172..0.227 rows=1 loops=1)
   Buffers: shared hit=37
   I/O Timings: read=0.000 write=0.000
   ->  Bitmap Index Scan using idx_member_roles_permissions  (cost=0.00..4.50 rows=1 width=0) (actual time=0.066..0.066 rows=108 loops=1)
         Index Cond: (member_roles.permissions @> '{"admin_runners": true}'::jsonb)
         Buffers: shared hit=5
         I/O Timings: read=0.000 write=0.000

Time: 1.606 ms
  - planning: 1.310 ms
  - execution: 0.296 ms
    - I/O read: 0.000 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 37 (~296.00 KiB) from the buffer pool
  - reads: 0 from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/30608/commands/94927