Add API for retrieving limit exclusions

What does this MR do and why?

Adds an admin-only API endpoint for retrieving all Namespaces::Storage::LimitExclusions records

• GET /namespaces/storage/limit_exclusions

This endpoint will be used by an admin-only frontend UI as part of https://gitlab.com/groups/gitlab-org/-/epics/9173

Testing

Replace :id and :your-pat (for an admin user) in the below cURLs accordingly:

First create some exclusions

curl --request POST \
  --url http://gdk.test:3000/api/v4/namespaces/:id/storage/limit_exclusion \
  --header 'Content-Type: application/json' \
  --header 'PRIVATE-TOKEN: :your-pat' \
  --data '{
	"reason": "testing an exclusion"
}'

Then retrieve all of them:

curl --request GET \
  --url http://gdk.test:3000/api/v4/namespaces/storage/limit_exclusions \
  --header 'PRIVATE-TOKEN: :your-pat' 

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.