feat(next)!: allows auth strategies to return headers that need to be… (#6964)

## Description

Some authentication strategies may need to set headers for responses,
such as updating cookies via a refresh token, and similar. This PR
extends Payload's auth strategy capabilities with a manner of
accomplishing this.

This is a breaking change if you have custom authentication strategies
in Payload's 3.0 beta. But it's a simple one to update.

Instead of your custom auth strategy returning the `user`, now you must
return an object with a `user` property.

This is because you can now also optionally return `responseHeaders`,
which will be returned by Payload API responses if you define them in
your auth strategies. This can be helpful for cases where you need to
set cookies and similar, directly within your auth strategies.

Before: 

```ts
return user
```

After:

```ts
return { user }
```

docs/authentication/custom-strategies.mdx

@@ -33,10 +33,12 @@ The `authenticate` function is passed the following arguments:
 
 ### Example Strategy
 
-At its core a strategy simply takes information from the incoming request and returns a user. This is exactly how Payloads built-in strategies function.
+At its core a strategy simply takes information from the incoming request and returns a user. This is exactly how Payload's built-in strategies function.
+
+Your `authenticate` method should return an object containing a Payload user document and any optional headers that you'd like Payload to set for you when we return a response.
 
 ```ts
-import { CollectionConfig } from 'payload/types'
+import { CollectionConfig } from 'payload'
 
 export const Users: CollectionConfig = {
   slug: 'users',
@@ -59,7 +61,18 @@ export const Users: CollectionConfig = {
             },
           })
 
-          return usersQuery.docs[0] || null
+          return {
+            // Send the user back to authenticate,
+            // or send null if no user should be authenticated
+            user: usersQuery.docs[0] || null,
+
+            // Optionally, you can return headers
+            // that you'd like Payload to set here when
+            // it returns the response
+            responseHeaders: new Headers({
+              'some-header': 'my header value'
+            })
+          }
         }
       }
     ]