Add caching support for assets served from the "public/" directory

[bitspittle]

Probably need a way to choose a default value for all assets with the ability to configure caching headers for one-off cases.

For reference:

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
• https://ktor.io/docs/caching.html
• https://nextjs.org/docs/going-to-production#caching