Only add x-fah-middleware header to routes that have middleware enabled

[mathu97]

Currently x-fah-middleware header is being added to all routes of an app that has a middleware file.

This PR updates the logic to only add x-fah-middleware header to routes that have middleware enabled. This is done by reading the middleware-manifest.json to get the routes that middleware is enabled on, and updating the routes-manifest.json file with the x-fah-middleware headers.

Testing this change locally:

1. Added a middleware.ts file that only matches the /blah route.

3. Build the app with the NextJs Adapter
4. Ran the app and saw the x-fah-middleware header only being added to the /blah route

[annajowang]

lgtm from me on good looking code