QnA: How to assert custom headers of the response?

Yes, just keep them under "headers" section as below(you can choose the fields to keep/skip, order doesn't matter).

e.g.

            "assertions": {
                "status": 200,
                "headers" : {
                    "X-Server-Token": 190215, //<--- e.g. If this is random or indeterministic, then use "$NOT.NULL"
                    "Server" : [ "hsbcbank.co.uk" ],
                    "DateTime" : "$NOT.NULL" //<--- Not null due to indeterministic for every response 
                },
                "body": {
                    "login" : "octocat",
                    "id" : 583231
                }
            }

In case you want to use a custom header token for your next step, then just point your JSON path to that header. e.g.

${$.create_emp.response.headers.X-Server-Token} //<--- This will resolve to 190215