Allow adding headers to requests (e.g. for Key Auth)

Problem/Motivation

When using Key Auth (https://www.drupal.org/project/key_auth), API request don't return data unless a correct key is provided via the header.

Steps to reproduce

Try running JSON:API Boost on a site with Key Auth enabled. The logs are full of "Access Denied" ... "The current user is not allowed to GET the selected resource."

Proposed resolution

Allow specifying a header for the requests made by JSON:API Boost. Users will be able to specify the key name and value from configured in Key Auth. The requests will succeed and warming will be achieved.

User interface changes

Provide a form to specify the name and value of a header to send with the API warming requests.