Ability to set custom headers

Added return value to hook function passed in configuration object. From now on, this function can return Map containing custom headers that will be set before sending response with open-graph image to client.

Example code:

import { withOGImage } from 'next-api-og-image'

export default withOGImage({
  template: {
    react: ({ myQueryParam }) => <div>🔥 {myQueryParam}</div>,
  },
  hook: () => {
    const headers = new Map()
    headers.set('X-My-Header', 'cool-thing')
    return headers
  },
})

Full Changelog: 4.1.0...4.2.0