feat: export cacheHeaders method

[eduardoboucas]

Exports the cacheHeaders method that returns a set of cache headers based on the settings supplied.

Example

import { cacheHeaders, DAY } from "@netlify/cache"

export default async () => new Response("I shall be cached", {
  headers: {
    ...cacheHeaders({
      ttl: 2 * DAY, // Two days
      tags: ["product", "sale"],
      vary: {
        cookie: ["ab_test_name", "ab_test_bucket"],
        query: ["item_id", "page"]
      }
    })
  }
})

[JakeChampion]

perhaps we should rename the method to netlifyCacheHeaders as it is not a generic cache headers function, it generates the netlify-cdn-cache-control, netlify-vary, netlify-cache-tag, and netlify-cache-id headers - what do you think?

[eduardoboucas]

Great question. I'm a bit torn, to be honest. I don't think the goal of this package is to be a set of utility methods for HTTP caching. There are better alternatives like https://github.com/ascorbic/cache-primitives for that. I think the main use case is to provide a set of utility methods for working with the Netlify cache, and in that sense I think netlifyCacheHeaders is a bit redundant, as we'd also need to prefix setCacheHeaders (which we're currently not documenting) and potentially others, making the names more verbose and clunkier.

[JakeChampion]

Suggested change

	export { cacheHeaders, setCacheHeaders } from './cache-headers/cache-headers.js'
	export { cacheHeaders as netlifyCacheHeaders, setCacheHeaders } from './cache-headers/cache-headers.js'