feat: add the ability to pass in a response to upload handlers

[JarrodMFlesch]

Description

Adds the ability to set response headers by using a new uploads.modifyResponseHeaders property. You could previously do this in Express in Payload v2.

You can do this like so:

upload: {
  modifyResponseHeaders: ({ headers }) => {
    headers.set('Cache-Control', 'public, max-age=86400')
    return headers
  }
},

• [x] I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] Existing test suite passes locally with my changes
• [ ] I have made corresponding changes to the documentation