[Proposal] public_file_server to support redirects

In the process of migrating a Rails application (e.g. myapp.com) from serving public static assets to external static asset host (e.g. cdn.myapp.com), I learned that we can’t immediately disable public_file_server configuration after setting asset_host to external file server because there is still some traffic for assets to the Rails app coming from emails sent prior to the asset host migration (and they refer to myapp.com/packs/...).

To ease this migration, I would love to have configuration such that:

if `config.asset_host` is not nil
and `config.public_file_server.enabled` is true
then `/packs/*` requests will receive with HTTP 301 to `config.asset_host`

The scenario as described above would be a breaking change behavior, which I don’t have strong opinions on. I am happy to discuss different ways to configure, but I would like to know if there is interest in having this feature to be built in Rails. Certainly happy to contribute the code pull request as well!