Add missing fields to NextConfig type (#27974)

This PR adds the `poweredByHeader` and `webpack` fields to the public `NextConfig` type.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes

packages/next/build/entries.ts

@@ -91,7 +91,7 @@ export function createEntrypoints(
     buildId,
     assetPrefix: config.assetPrefix,
     generateEtags: config.generateEtags ? 'true' : '',
-    poweredByHeader: config.poweredByHeader,
+    poweredByHeader: config.poweredByHeader ? 'true' : '',
     canonicalBase: config.amp.canonicalBase || '',
     basePath: config.basePath,
     runtimeConfig: hasRuntimeConfig

packages/next/server/config-shared.ts

@@ -48,6 +48,22 @@ export type NextConfig = { [key: string]: any } & {
   webpack5?: false
   excludeDefaultMomentLocales?: boolean
 
+  webpack?:
+    | ((
+        config: any,
+        context: {
+          dir: string
+          dev: boolean
+          isServer: boolean
+          buildId: string
+          config: NextConfigComplete
+          defaultLoaders: { babel: any }
+          totalPages: number
+          webpack: any
+        }
+      ) => any)
+    | null
+
   trailingSlash?: boolean
   env?: { [key: string]: string }
   distDir?: string
@@ -58,6 +74,7 @@ export type NextConfig = { [key: string]: any } & {
   generateEtags?: boolean
   pageExtensions?: string[]
   compress?: boolean
+  poweredByHeader?: boolean
   images?: ImageConfig
   devIndicators?: {
     buildActivity?: boolean