feature: add withCredentials configuration key (via #5149)

* Add the withCredentials configuration key

It enables passing credentials in CORS requests. e.g. Cookies and
Authorization headers.

* Improve withCredentials documentation

* Add unit tests for the withCredentials config

* Update configuration.md

* Update configuration.md

* only set `withCredentials` Fetch flag if the config value is truthy

there are some workarounds in the wild today that involve setting `withCredentials` on `system.fn.fetch` directly. 

this approach avoids mangling those existing workarounds!

* add more test cases

* Update configs-wrap-actions.js

* Update index.js

docker/configurator/variables.js

@@ -86,6 +86,10 @@ const standardVariables = {
   VALIDATOR_URL: {
     type: "string",
     name: "validatorUrl"
+  },
+  WITH_CREDENTIALS: {
+    type: "boolean",
+    name: "withCredentials",
   }
 }
 
@@ -102,4 +106,4 @@ const legacyVariables = {
   }
 }
 
-module.exports = Object.assign({}, standardVariables, legacyVariables)
+module.exports = Object.assign({}, standardVariables, legacyVariables)