make "require subscription" toggleable in build (#6144)

## Summary

Adds build time feature flags system starting with a flag that indicates
whether subscription is required to use the app. This is only used on
cloud.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6144-make-require-subscription-toggleable-in-build-2916d73d3650813bb140c5e96bcce1ce)
by [Unito](https://www.unito.io)

src/platform/cloud/subscription/composables/useSubscription.ts

@@ -26,7 +26,7 @@ interface CloudSubscriptionStatusResponse {
 const subscriptionStatus = ref<CloudSubscriptionStatusResponse | null>(null)
 
 const isActiveSubscription = computed(() => {
-  if (!isCloud) return true
+  if (!isCloud || !__BUILD_FLAGS__.REQUIRE_SUBSCRIPTION) return true
 
   return subscriptionStatus.value?.is_active ?? false
 })