No availability to send json response for specific .wellk-nown items.

Problem/Motivation

Some ".well-known" item should send "application/json" requests.
Moreover, in some language config, the well-known route will be redirected to an url with language prefix.

Steps to reproduce

1. Go to the config form : /admin/config/development/well-known
2. Add a new well-known path with json content :
- name : assetlinks.json
- value : [{"type":"json"}]
3. Call the url /.well-known/assetlinks.json
4. the response header "content-type" should be "application/json"

Proposed resolution

- Add the availability to define the type of response for each .well-known item.
- Avoid the language prefix redirection.