feat(server-auth): dbAuth 3/3 - handle login, logout, signup, etc. requests if forwarded from middleware

.changesets/10457.md

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+- feat(server-auth): dbAuth 3/3 -  handle login, logout, signup, etc. requests if forwarded from middleware (#10457) by @dac09
+
+This PR updates the DbAuthHandler class to handle requests forwarded from middleware, so it can generate responses for login, logout, signup, etc. These are POST requests - it used to be to the `/auth` function, but now they will be captured by dbAuth middleware and forwarded onto DbAuthHandler. 
+
+**High level changes:**
+- use the `Headers` class in each of the "method" responses. This allows us to set multi-value headers like Set-Cookie. A simple object would not. See type `AuthMethodOutput`
+- extracts `buildResponse` into a testable function and adds test. For `Set-Cookie` headers we return an array of strings. 
+
+In the middleware here's the code I had for the final conversion:
+```ts
+    if (AUTHHANDLER_REQUEST) {
+      const output = await dbAuthHandler(req)
+
+      const finalHeaders = new Headers()
+      Object.entries(output.headers).forEach(([key, value]) => {
+        if (Array.isArray(value)) {
+          value.forEach((v) => finalHeaders.append(key, v))
+        } else {
+          finalHeaders.append(key, value)
+        }
+      })
+
+      return new MiddlewareResponse(output.body, {
+        headers: finalHeaders,
+        status: output.statusCode,
+      })
+    }
+```