* refactor: incorporate custom headers into default header setup
Modify the default headers construction to merge in custom headers if provided. This change ensures that custom headers, such as Cloudflare access tokens, are included in the request before self.get_token() is called and without overwriting the default headers like 'Authorization' and 'Content-Type'.
* refactor: Fixing linting issue
Fixing linting issue by adding trailing comma to key in dict
* refactor: adding custom headers to KeycloakOpenID
Adding `custom_headers` to KeycloakOpenID instantiation.