Refactor Supabase client and service role key usage

The refactoring includes the moving of the `getSupabaseClientKeys()` and `getServiceRoleKey()` function calls to the module scope and the addition of a `warnServiceRoleKeyUsage()` function, used to display a warning when the Supabase Service Role is accessed. The change aims to improve code readability and maintainability.

packages/supabase/src/get-service-role-key.ts

@@ -1,3 +1,5 @@
+import 'server-only';
+
 import { z } from 'zod';
 
 /**
@@ -7,12 +9,16 @@ import { z } from 'zod';
  */
 export function getServiceRoleKey() {
   const serviceRoleKey = process.env.SUPABASE_SERVICE_ROLE_KEY;
+  return z.string().min(1).parse(serviceRoleKey);
+}
 
+/**
+ * Displays a warning message if the Supabase Service Role is being used.
+ */
+export function warnServiceRoleKeyUsage() {
   if (process.env.NODE_ENV !== 'production') {
     console.warn(
       `[Dev Only] This is a simple warning to let you know you are using the Supabase Service Role. Make sure it's the right call.`,
     );
   }
-
-  return z.string().min(1).parse(serviceRoleKey);
 }