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

test/core/plugins/swagger-js/withCredentials.js

@@ -0,0 +1,94 @@
+import expect, { createSpy } from "expect"
+import { loaded } from "corePlugins/swagger-js/configs-wrap-actions"
+
+describe("swagger-js plugin - withCredentials", () => {
+  it("should have no effect by default", () => {
+    const system = {
+      fn: {
+        fetch: createSpy().andReturn(Promise.resolve())
+      },
+      getConfigs: () => ({})
+    }
+    const oriExecute = createSpy()
+
+    const loadedFn = loaded(oriExecute, system)
+    loadedFn()
+
+    expect(oriExecute.calls.length).toBe(1)
+    expect(system.fn.fetch.withCredentials).toBe(undefined)
+  })
+
+  it("should allow setting flag to true via config", () => {
+    const system = {
+      fn: {
+        fetch: createSpy().andReturn(Promise.resolve())
+      },
+      getConfigs: () => ({
+        withCredentials: true
+      })
+    }
+    const oriExecute = createSpy()
+
+    const loadedFn = loaded(oriExecute, system)
+    loadedFn()
+
+    expect(oriExecute.calls.length).toBe(1)
+    expect(system.fn.fetch.withCredentials).toBe(true)
+  })
+  
+  it("should allow setting flag to false via config", () => {
+    const system = {
+      fn: {
+        fetch: createSpy().andReturn(Promise.resolve())
+      },
+      getConfigs: () => ({
+        withCredentials: false
+      })
+    }
+    const oriExecute = createSpy()
+
+    const loadedFn = loaded(oriExecute, system)
+    loadedFn()
+
+    expect(oriExecute.calls.length).toBe(1)
+    expect(system.fn.fetch.withCredentials).toBe(false)
+  })
+  
+   it("should allow setting flag to true via config as string", () => {
+    // for query string config
+    const system = {
+      fn: {
+        fetch: createSpy().andReturn(Promise.resolve())
+      },
+      getConfigs: () => ({
+        withCredentials: "true"
+      })
+    }
+    const oriExecute = createSpy()
+
+    const loadedFn = loaded(oriExecute, system)
+    loadedFn()
+
+    expect(oriExecute.calls.length).toBe(1)
+    expect(system.fn.fetch.withCredentials).toBe(true)
+  })
+  
+  it("should allow setting flag to false via config as string", () => {
+    // for query string config
+    const system = {
+      fn: {
+        fetch: createSpy().andReturn(Promise.resolve())
+      },
+      getConfigs: () => ({
+        withCredentials: "false"
+      })
+    }
+    const oriExecute = createSpy()
+
+    const loadedFn = loaded(oriExecute, system)
+    loadedFn()
+
+    expect(oriExecute.calls.length).toBe(1)
+    expect(system.fn.fetch.withCredentials).toBe(false)
+  })
+})