Add index for member_namespace and related columns

What does this MR do and why?

In !105781 (merged) we are adding a set of new Project and Group member relationships. This index supports those new relationships.

CREATE INDEX CONCURRENTLY "index_members_on_member_namespace_id_compound" ON "members" (
  "member_namespace_id", "type", "requested_at", "id"
)

SQL Query Plans

15846663 is the ID of the project namespace for the gitlab project.

9970 is the gitlab-org group

project.namespace_members

SELECT 
  "members".* 
FROM 
  "members" 
WHERE 
  "members"."type" = 'ProjectMember' 
  AND "members"."member_namespace_id" = 15846663 
  AND "members"."requested_at" IS NULL

Query plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14090/commands/49219

---

project.namespace_requesters

SELECT 
  "members".* 
FROM 
  "members" 
WHERE 
  "members"."type" = 'ProjectMember' 
  AND "members"."member_namespace_id" = 15846663 
  AND "members"."requested_at" IS NOT NULL

Query plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14090/commands/49220

---

group.namespace_members

SELECT 
  "members".* 
FROM 
  "members" 
WHERE 
  "members"."type" = 'GroupMember' 
  AND "members"."member_namespace_id" = 9970 
  AND "members"."requested_at" IS NULL 
  AND "members"."access_level" != 5

Query plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14090/commands/49221

---

group.namespace_requesters

SELECT 
  "members".* 
FROM 
  "members" 
WHERE 
  "members"."type" = 'GroupMember' 
  AND "members"."member_namespace_id" = 9970 
  AND "members"."requested_at" IS NOT NULL

Query plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/14090/commands/49222

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.