feat(customer-type): Add customer type, firstname and lastname to customer

[brunomiguelpinto]

Context

We currently only create Lago customers as companies, but there is a need to support both companies and individuals. This change is motivated by scenarios where customers may be a mix of B2B and B2C, and where external integrations require handling both Contacts and Companies.

To address this, we are introducing a new field, customer_type, to distinguish whether a customer is a company or an individual. Existing customers will remain unaffected with customer_type set to the default nil.

Description

This PR adds firstname, lastname and customer_type to the customer object and create input.