Add an API to resync the public folder

[gzhhong]

Goals

1. To add an API to resync the public folder.

Non-Goals

Background

The content in public folder are static and will not be able to be changed dynamically after the nextjs app started. I.e. when a new image added to the pulbic folder, it is not accessible outside the nextjs app. It will be helpful if nextjs provide an API to 'refresh' public folder after content changed.

Proposal

To add a new API to refresh public folder