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