Refactor route handlers and CMS clients

Refactored the route handlers to use a new `enhanceRouteHandler` function for better control over request handlers and user authentication. CMS clients are now created using factory functions for better encapsulation and control over instance creation. Renamed `client.ts` in 'keystatic' to `keystatic-client.ts`.

packages/cms/keystatic/src/index.ts

@@ -1,2 +1,2 @@
-export * from './client';
+export * from './keystatic-client';
 export * from './content-renderer';