DEV: Add tests to S3Helper.ensure_cors and move rules to class (#14767)

In preparation for adding automatic CORS rules creation
for direct S3 uploads, I am adding tests here and moving the
CORS rule definitions into a dedicated class so they are all
in the one place.

There is a problem with ensure_cors! as well -- if there is
already a CORS rule defined (presumably the asset one) then
we do nothing and do not apply the new rule. This means that
the S3BackupStore.ensure_cors method does nothing right now
if the assets rule is already defined, and it will mean the
same for any direct S3 upload rules I add for uppy. We need
to be able to add more rules, not just one.

This is not a problem on our hosting because we define the
rules at an infra level.

lib/backup_restore/s3_backup_store.rb

@@ -83,14 +83,7 @@ module BackupRestore
     end
 
     def ensure_cors!
-      rule = {
-        allowed_headers: ["*"],
-        allowed_methods: ["PUT"],
-        allowed_origins: [Discourse.base_url_no_prefix],
-        max_age_seconds: 3000
-      }
-
-      @s3_helper.ensure_cors!([rule])
+      @s3_helper.ensure_cors!([S3CorsRulesets::BACKUP_DIRECT_UPLOAD])
     end
 
     def cleanup_allowed?