feat(credit_notes): Improve credit note schema

[vincent-pochet]

Roadmap Task

👉 getlago/lago#59

Context

The objective of this feature is to introduce a new concept of credit notes.

The main reason behind this need is to handle the following case:

• If an overdue has been paid because a customer passes from a yearly plan to a monthly plan (in_advance), we charge the customer as if the remainder does not exist.

Description

This PR Improves the currently merged credit note features:

• Adds a refunded_at fields to credit note
• Removes refund_amount_cents and refund_amount_currency from credit note item
• Renames credit_amount_cents and credit_amount_currency into amount_*