Add API for creating limit exclusions

What does this MR do and why?

Adds an admin-only API endpoint for creating Namespaces::Storage::LimitExclusions records relating to a Namespace

• POST /namespaces/:id/storage/limit_exclusion - create a new exclusion

Eventually we'll also add DELETE and GET endpoints (!116531 (merged), !116532 (merged) - separate MRs to keep the diff easier to review)

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

Docs preview: https://gitlab.com/gitlab-org/gitlab/-/blob/dbbd9ada6492d2410784549cda1bba32bc9ed6d0/doc/development/internal_api/index.md#storage-limit-exclusions

Testing

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

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"
}'

Refs https://gitlab.com/gitlab-org/gitlab/-/issues/385740

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.