update subscription with metered products at trial ending (#11319)

Context
- Subscription with metered prices can't be 'paused' at the end of
trialing period
- Currently, pausing subscription have been the process we choose at
Twenty

Two solutions : 
- [x] (The chosen one!) Adding metered products when the trial period is
ended.
- [ ] Switching from 'paused' to 'past_due' status at the end of
trialing period. Tricky because we should handle different cases of
'past_due' subscription status, some causing workspace suspension and
some other not.

closes https://github.com/twentyhq/core-team-issues/issues/676

packages/twenty-server/src/engine/core-modules/feature-flag/enums/feature-flag-key.enum.ts

@@ -13,4 +13,5 @@ export enum FeatureFlagKey {
   IsNewRelationEnabled = 'IS_NEW_RELATION_ENABLED',
   IsWorkflowFormActionEnabled = 'IS_WORKFLOW_FORM_ACTION_ENABLED',
   IsPermissionsV2Enabled = 'IS_PERMISSIONS_V2_ENABLED',
+  IsMeteredProductBillingEnabled = 'IS_METERED_PRODUCT_BILLING_ENABLED',
 }