add fetch billing products from tables instead of env variables (#9601)

Solves https://github.com/twentyhq/private-issues/issues/237

**TLDR:**

- Fetches billing products and prices from the tables BilllingProducts
and BillingPrices instead of fetching the product from the environment
variables and the prices from the stripe API.
- Adds new feature flag for this feature
- Fixes calls used to fetch stripe products and prices for the command
Billing Sync Plans Data.


**In order to test:**

1. Have the environment variable IS_BILLING_ENABLED set to true and add
the other required environment variables for Billing to work
2. Do a database reset (to ensure that the new feature flag is properly
added and that the billing tables are created)
3. Run the command: `npx nx run twenty-server:command
billing:sync-plans-data` (if you don't do that the products and prices
will not be present in the database)
4. Run the server , the frontend, the worker, and the stripe listen
command (`stripe listen --forward-to
http://localhost:3000/billing/webhooks`)
5. Buy a subscription for the Acme workspace and play with the project

**Doing**

I think there is some room of progress for the function
formatProductPrices, I used a similar version that was done before, I'll
look into that.

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

@@ -15,4 +15,5 @@ export enum FeatureFlagKey {
   IsCommandMenuV2Enabled = 'IS_COMMAND_MENU_V2_ENABLED',
   IsJsonFilterEnabled = 'IS_JSON_FILTER_ENABLED',
   IsLocalizationEnabled = 'IS_LOCALIZATION_ENABLED',
+  IsBillingPlansEnabled = 'IS_BILLING_PLANS_ENABLED',
 }