feat: move custom image cdn url generator implementation to adapter

[pieh]

Description

This builds on top of #38685 to add pluggable way of providing custom image cdn url generator implementation by adapters, so platform specific endpoint is not hardcoded in platform agnostic package

Documentation

Tests

Related Issues

FRA-95

[pieh]

Some context here why fileCDN and placeholders are not checked - simply they don't exactly work.

fileCDN explanation:
Because fileCDN is not supported - it would try to run FILE_CDN job "locally" in lambda - however there is a problem with running actual IMAGE_CDN and FILE_CDN jobs (this happens when there is no handling at request time) - the "worker" responsible for handling those jobs is failing to load - this happens locally as well in gatsby serve, so this is not adapter or Netlify issue - it's a framework issue. However even if we fix worker loading - it still actually wouldn't exactly "just work" on Netlify as there is no straight forward way to add additional static assets to deployment after initial build and deployment finished - this likely could be supported via proxy lambda or something like that, but that would need additional work to properly work.

placeholders explanation:
linkfs we use in

gatsby/packages/gatsby/src/utils/page-ssr-module/lambda.ts

Lines 53 to 72 in e3365ab

	// Alias the cache dir paths to the temp dir
	const lfs = link(fs, rewrites) as typeof import("fs")
	// linkfs doesn't pass across the `native` prop, which graceful-fs needs
	for (const key in lfs) {
	if (Object.hasOwnProperty.call(fs[key], `native`)) {
	lfs[key].native = fs[key].native
	}
	}
	const dbPath = path.join(TEMP_CACHE_DIR, `data`, `datastore`)
	// 'promises' is not initially linked within the 'linkfs'
	// package, and is needed by underlying Gatsby code (the
	// @graphql-tools/code-file-loader)
	lfs.promises = link(fs.promises, rewrites)
	// Gatsby uses this instead of fs if present
	// eslint-disable-next-line no-underscore-dangle
	global._fsWrapper = lfs

doesn't seem to provide implementation for new lfs.WriteStream() which is used to generate placeholders (it does provide implementation for fs.createWriteStream so it should be possible to fix/patch it up, but didn't want to do it in this PR)

[kathmbeck]

I wonder if we want to add any kind of comment here that we are using a field that is supported but not documented with crop (position is documented but both work with the api).

[pieh]

is it just alias and we could just use position instead of crop to use documented one?

(I just moved implementation as-is from gatsby-plugin-utils alternative url generator, so I have no context on this part)

[kathmbeck]

Yes that's a good point. I was trying to mirror using the same parameters, but I suppose we can just change it.