This adds provisioning endpoints for downloading alert rules and alert rule groups in a
format that is compatible with file provisioning. Each endpoint supports both json and
yaml response types via Accept header as well as a query parameter
download=true/false that will set Content-Disposition to recommend initiating a download
or inline display.
This also makes some package changes to keep structs with potential to drift closer
together. Eventually, other alerting file structs should also move into this new file
package, but the rest require some refactoring that is out of scope for this PR.
