Allow plugins to overwrite the cachedRead function

[SimonSiefke]

Is your feature request related to a problem? Please describe.
I would like to overwrite the cachedRead function for a VSCode plugin for Vite https://github.com/SimonSiefke/vscode-vite. The idea is to not serve the files from the filesystem but directly from VSCode so that one can see the page update while typing. The prototype is already working but I basically needed to fork Vite only to modify the readFile behaviour. That makes it difficult to pull in upstream changes from Vite.

Describe the solution you'd like
A possible solution would be to put the cachedRead function into the context object:

// node/server/index.ts
import { cachedRead } from '../utils'

const context: Context = {
    root,
    app,
    server,
    watcher,
    resolver,
    config,
    read: cachedRead // put cachedRead onto the context
}

// serverPluginEsBuild.ts
if (ctx.path === vueJsxPublicPath) {
  await ctx.read(ctx, vueJsxFilePath) // use ctx.read instead of cachedRead
}

// serverPluginServeStatic.ts
const filePath = resolver.requestToFile(ctx.path)
if (
  filePath !== ctx.path &&
  fs.existsSync(filePath) &&
  fs.statSync(filePath).isFile()
   ) {
  await ctx.read(ctx, filePath) // use ctx.read instead of cachedRead
}

// serverPluginVue.ts
const importer = ctx.path
const importee = cleanUrl(resolveImport(root, importer, block.src!, resolver))
const filePath = resolver.requestToFile(importee)
await ctx.read(ctx, filePath) // use ctx.read instead of cachedRead
block.content = ctx.body

And plugins will be able to overwrite the read function:

const myPlugin: ServerPlugin = (ctx) => {
  ctx.read = newRead
}

Additional context

[yyx990803]

Cool idea! ctx.read sounds reasonable. Open to a PR.