Automatic customer generation

[hellopablo]

Consider a [configurable] system to automatically map users with customers. e.g.:

• Add customer_id column to user meta as part of the invoice module migration
• Listen for user creation and create or link customers
• Listen for user updates and update customers

⚠️ This will need to be optional as not all systems have a 1:1 requirement with users and customers. Perhaps this can be configured to listen for a particular model's create/update. e.g. a Business model.