function(creds) {
  var temp = creds.username + ':' + creds.password;
  var Base64 = Java.type('java.util.Base64');
  var encoded = Base64.getEncoder().encodeToString(temp.bytes);
  return {Authorization:  'Basic ' + encoded};
}

...
config.apitester1AuthHeader =
karate.callSingle('classpath:auth-headers.js', {username:'apitester1', password:'xxx'});
config.apitester2AuthHeader =
karate.callSingle('classpath:auth-headers.js', {username:'apitester2', password:'xxx'});
...

Feature: project end-point

Background:
* url baseUrl
Scenario: get projects user has right to
  * header = apitester1AuthHeader
  Given path 'project'
...