Provide ability to connect to password-protected SPARQL endpoints

[tfrancart]

It is possible to add an Authentication header in YASGUI:

      const yasqe = new Yasqe(document.getElementById("yasqe"), {
        requestConfig: { 
            endpoint: $('#endpoint').text()

            // headers: () => ({
            //  'Authorization': 'Basic ' + btoa(unescape(encodeURIComponent('LOGIN' + ':' + 'PASSWORD')))
            // })
        },
        copyEndpointOnNewTab: false  
      });

But how do we provide the same header to the autocomplete and list handlers ?

[tfrancart]

Done in #517