feat: adds and exports reusable auth server functions (#11900)

### What
Adds exportable server functions for `login`, `logout` and `refresh`
that are fully typed and ready to use.

### Why
Creating server functions for these auth operations require the
developer to manually set and handle the cookies / auth JWT. This can be
a complex and involved process - instead we want to provide an option
that will handle the cookies internally and simplify the process for the
user.

### How
Three re-usable functions can be exported from
`@payload/next/server-functions`:
- login
- logout
- refresh

Examples of how to use these functions will be added to the docs
shortly, along with more in-depth info on server functions.

test/server-functions/components/refresh.tsx

@@ -0,0 +1,7 @@
+'use client'
+import { refreshFunction } from './refreshFunction.js'
+
+const RefreshToken = () => {
+  return <button onClick={() => refreshFunction()}>Custom Refresh</button>
+}
+export default RefreshToken