From a Twig template, how to retrieve the content of a HTTP header in the current request? I would like to detect if a header in the current request is present and obtain its value, directly within a Twig template. 
I know that craft.app.request gives access to a few variables like isAjax that directly check the headers content, but I haven't found a method to directly retrieve a specific header.

Can this be achieved in some way? (preferably not by writing a plugin)
By checking the code, I could find a method named getHeaders() that will return all the headers of the current request.

So if you want to retrieve the content of the foo header, just use:

{% set headers = craft.app.request.getHeaders() %}
{% set fooHeader = headers['foo'] %}